(FORTUNE Magazine) – Even if a proposed new rule by the Federal Reserve were already in effect, the Fed wouldn't be able to lay a glove on General Electric's purchase of RCA for $6.28 billion in cash. But after New Year's Day, deals might fall through if financed like many corporate takeovers in recent years -- by low-rated junk bonds, issued by shell corporations and backed by the target company's stock. In a close vote the Fed proposed that in such transactions the target company's stock secure no more than 50% of the purchase price. The proposal is a new interpretation of the old restrictions on buying stocks on margin. In force since 1934, the margin limitation was designed to curb speculative binges on the stock market. In its new guise it is widely interpreted as an attempt to control the vast amount of debt accumulated during the current merger mania -- debt that Fed Chairman Paul A. Volcker / finds incompatible with long-term economic and financial stability. Drexel Burnham Lambert, the New York-based brokerage that dominates the huge junk- bond market ($15 billion so far this year), took issue with the ruling. Said Frederick H. Joseph, Drexel's C.E.O.: ''There is no evidence that these financings (junk bonds) are harmful to the financial system.'' The new limitations could take the leverage out of some leveraged takeovers and give worried C.E.O.s a little peace of mind. Corporate raiders like T. Boone Pickens and Carl Icahn may find it more difficult to finance their campaigns. David-size companies will be less likely to swallow up industrial Goliaths. But the ruling limits the way in which takeovers are financed, not takeovers themselves. Acquiring companies may have to put their own credit on the line to raise cash, or they may become more creative. ''There are a lot of bright, imaginative people in this business,'' says Roger Miller, managing director of Salomon Brothers' mergers and acquisitions department. ''They will immediately go into a reactive mode and find other ways of doing things.''