(FORTUNE Magazine) – Strange food, hostile natives, the appalling difficulty of getting the sports scores: From Marco Polo to today's road warriors, doing business overseas has never been easy. But at least Marco Polo didn't have to worry about trial lawyers.

Relying on the once-obscure Alien Tort Statute of 1789, trial lawyers have found a new way to make life miserable for business. Since the early 1990s about two dozen companies have been sued for alleged complicity in abuses committed overseas--from torture in Guatemala (Del Monte) to murder in Colombia (Coke) to environmental harm in Ecuador (Texaco). No one, of course, wants to defend corporate wrongdoing. But the statute is being stretched so that firms operating in a normal and legal manner--albeit in a rotten place, such as apartheid South Africa--also have reason to fear being sued.

No cases have yet gone to trial. But on June 17, the U.S. Court of Appeals for the Second Circuit will decide whether a suit against Unocal for alleged human rights abuses in Burma can proceed. No one says that employees of the U.S. oil firm wanted or imposed the use of forced labor--an abuse that did indeed occur on a pipeline project in which Unocal was a minority investor. But a three-judge panel ruled last September that just because Unocal didn't commit the abuse doesn't mean it wasn't responsible. "Knowing practical assistance [as opposed to unintentional assistance] or encouragement that has a substantial effect on the perpetration of the crime" is enough to trigger liability, said the panel. The full appeals court is reconsidering that decision.

Business groups are also closely watching several suits against companies for alleged complicity in apartheid--including one brought by Edward Fagan, a lawyer best known for his involvement with Holocaust-related litigation; and one by Michael Hausfeld, a Washington lawyer acting for a South African victims' group. Hausfeld's complaint names 20 companies. For example, it charges that Swiss bank UBS helped prop up apartheid by making loans to the government and that Exxon did the same by selling oil to the military. Ditto for Shell and BP. Hausfeld says the companies are not being sued "merely for doing business, but for doing business which knowingly aids and abets the furtherance of the commission of a human rights abuse." Under that standard, though, just about any commerce with the apartheid government could fit the bill, which explains why such unlikely suspects as Johnson & Johnson and Sara Lee have also been sued. President Mbeki of South Africa calls the suits "completely unacceptable."

The basis for these claims, the 1789 Alien Tort Statute, reads: "The district courts shall have original jurisdiction of any civil action by an alien for a tort only, committed in violation of the law of nations or a treaty of the U.S." The statute was probably intended to signal that pirates could expect no safe haven in the new nation; for the better part of two centuries, it gathered dust. But since the 1980s, U.S. courts have steadily expanded the statute's range--first to include foreign governments' actions against their own citizens, then to private individuals, to U.S. corporations, and to non-U.S. companies. The idea of what constitutes a tort that violates the law of nations has also broadened. No one disputes that torture, genocide, and slavery qualify. But plaintiffs have argued that forced relocation, fraud, and breach of duty to treat with dignity should also be included. The courts have tossed out most (but not all) such claims. Fraud, for example, may be illegal everywhere, but it is not a violation of international law.

The apartheid cases are a test of whether the courts will rein in the statute. If the suits go forward, critics ask, what next? Will companies that invest in China be tarred with Beijing's human rights violations? Could alien torts become the next asbestos? Kenneth Roth, executive director of Human Rights Watch, thinks not. "This is a tiny, tiny issue," he insists. The statute is "absolutely a good thing. It is one of the few mechanisms available to hold abusive figures and corporations accountable." Problem is, until either Congress or the Supreme Court steps in, federal and state courts are creating this crucial area of law on the hoof--a decision here, an appeal there. The result is a muddle.

Even many critics of the recent suits don't mind the idea of U.S. courts adjudicating corporate horrors--and if that's Yankee unilateralism, so be it. What the law needs is clarity. Otherwise, think asbestos. --Cait Murphy