Interco Inc., the St. Louis conglomerate that went $1.65 billion into debt in a financial restructuring 2 1/2 years ago to fend off a hostile takeover attempt by Washington's Rales brothers, filed for bankruptcy protection yesterday.

At the same time, Interco sued the New York investment banking firm that put the restructuring together, Wasserstein Perella & Co., alleging that Interco was given poor advice when it sold its Ethan Allen furniture division for nearly $400 million to help pay off the debt.

Coincidentally, the Rales brothers agreed yesterday to pay a $850,000 penalty to settle Federal Trade Commission charges that they violated the Hart-Scott-Rodino antitrust act by buying more than $15 million in Interco stock in 1988 without notifying the FTC. The penalty was the third largest ever in such a case, but is merely a fraction of the estimated $75 million in profit that the brothers made on their Interco stock.

The flurry of legal activity marked a new chapter in the tangled saga of Interco, one of several late-1980s restructurings, leveraged buyouts and acquisitions that have collapsed into bankruptcy under the weight of huge debts and a faltering economy.

Interco was a sleepy conglomerate with annual revenue of $3.3 billion and interests from Converse sneakers to London Fog raincoats to Ethan Allen furniture when Steven and Mitchell Rales began acquiring stock in the company in 1988 through a firm called Equity Group Holdings. The Raleses, who control Danaher Corp., a District-based manufacturing conglomerate, acquired nearly 9 percent of the company and made several bids to buy the rest.

The FTC charged last year that Equity Group Holdings had violated the Hart-Scott-Rodino act by not promptly notifying the FTC that it had acquired more than $15 million of Interco stock. In addition, the FTC said, the Raleses continued to acquire Interco stock through other companies they set up without properly notifying the commission.

In settling the charges yesterday, Equity Group Holdings denied any wrongdoing, and said it settled to avoid protracted and expensive legal proceedings.

But sources said the settlement frustrated some FTC staff members and at least one commissioner, who had been hoping for a stiffer penalty, possibly including forfeiture of the Raleses' Interco profit.

To avoid the Raleses' takeover efforts, Interco underwent a $2.8 billion restructuring in late 1988, arranged by Wasserstein Perella.

But Interco quickly ran into financial problems when the operations it sold to help pay for the restructuring failed to bring as much money as hoped. Negotiations with bondholders and other creditors over the past year failed to win an agreement to refinance the debt, so the company sought protection from its creditors under Chapter 11 of the federal bankruptcy code in federal bankruptcy court in St. Louis. A company in Chapter 11 is shielded from its creditors while it continues to operate and draws up a plan to pay its debts.

A spokesman for the company said yesterday that Interco and its creditors "still are talking; they feel like some progress is being made," but the company wanted the shelter of Chapter 11 to complete the negotiations.

In its suit against Wasserstein Perella, Interco alleges that its problems were compounded by the investment bank's inability to get the most favorable price for Ethan Allen. The suit charges Wasserstein Perella with negligence and fraudulent misrepresentation, and asks damages of at least $89.5 million.

Wasserstein Perella, which earned fees of nearly $9 million for its work for Interco, issued a statement saying the allegations "are totally without merit."