(FORTUNE Magazine) – ANTIDOTES to takeover artists are becoming as popular as weedkiller in midsummer. Congress, which last year flirted with a law curbing greenmail --the purchase at a premium price of stock held by someone threatening to take over a company--is gearing up for new hearings. In January a Delaware court handed down a little-noted decision that Joseph Flom, whose law firm was on the losing side, calls ''one of the most fundamental changes in takeover law in the last 20 years.'' On another front, pension funds controlling $100 billion have formed a new organization, the Council of Institutional Investors, to oppose the payment of greenmail and other management actions that they say hurt shareholders. In the Delaware case the state's chancery court upheld the right of Household International, the parent of Household Finance, to issue so-called poison-pill warrants. These warrants--rights to buy stock--can poison a deal for anyone attempting a hostile takeover by ratcheting the cost skyward. Dreamed up by Martin Lipton, Joe Flom's frequent opponent in takeover battles, the device works like this: Household directors authorized warrants that stockholders could exercise if someone accumulated 20% of Household's shares. The warrants give stockholders the right to buy Household shares at about three times today's market price. Or--now watch for the cyanide, folks --if the hostile company merged with Household, the stockholders would have the right to buy the acquiring company's stock at half the market price. In the unlikely event that the aggressor went that far, it would take on Household's obligations and would have to honor the warrants, enriching warrant holders and making the cost of the takeover exorbitant. Flom's firm is appealing the Household ruling to the Delaware supreme court, but a few corporations have already stocked their medicine chests with poison pills. Among them: Colgate-Palmolive, Owens-Illinois, and Crown Zellerbach. The pension funds, mostly representing government workers, that have banded together in the Council of Institutional Investors would like to get lots more say about the way corporations react to takeover attempts. The council is a brainchild of California state treasurer Jesse Unruh. He is a trustee of two large pension funds, the California Public Employees' Retirement Fund (assets: $24 billion) and the California State Teachers' Retirement Fund (assets: $13.5 billion), that invest in many companies. The council, which was officially launched in January, now includes 20 of the nation's largest pension funds. Says New York City comptroller Harrison Goldin, a co-chairman of the group: ''We're sending a signal to corporate America that we will not sit idly by.'' The group's first target is Phillips Petroleum Co., which struck a peacemaking deal with T. Boone Pickens Jr. and now faces a new takeover threat from New York investor Carl C. Icahn. Phillips agreed to pay Pickens and his friends $53 a share, about a $6 premium over the market price. The council plans to meet with Pickens and Phillips's chairman, William Douce, before stockholders vote on a recapitalization plan forced on the company by Pickens. While the funds have a fiduciary responsibility to cast their votes individually, council members will caucus after their powwow with Douce and Pickens. Wall Street pros are worried. Says Ken Miller, head of Merrill Lynch's merger and acquisition group: ''If the institutions start speaking with one voice, they could become a financial OPEC.''