Bishop, Cook, Purcell & Reynolds, one of the District's home-grown regulatory law firms, has merged with 400-member Winston & Strawn of Chicago, and in the process lost two of its most prominent partners -- former U.S. attorney Joseph E. diGenova and firm chairman Wayne S. Bishop.

DiGenova, who joined Bishop, Cook in March 1988, has opted out of the merged firm, which will be one of the nation's largest with about 475 lawyers, because of a client conflict, he said.

DiGenova represents a cable industry trade association whose interests could conflict with those of clients already represented by Winston & Strawn, according to sources.

It is virtually impossible for a law firm to represent clients with competing interests.

DiGenova said the conflict was discovered after the two firms were well into merger talks.

"It's unfortunate and sad, but also a great opportunity," he said, adding that the discovery of such conflicts is "just one of the accidents of the merger process" and a reality of practicing law in the 1990s.

Bishop, who as chairman helped seek the merger, said Winston & Strawn "had all the ingredients we hoped for in a merger partner," except one: a strong ability to handle international financial transactions, which he said is an essential part of his international trade practice.

For that reason, he said, he plans to leave the firm and already is talking with a variety of law firms.

A group of lawyers doing communications work with Bishop, Cook also will depart because of conflicts with one of Winston & Strawn's clients, sources said.

Sources said both diGenova and Bishop are talking with a number of firms, including Manatt, Phelps, Rothenberg & Phillips, whose Washington office is home to Charles T. Manatt, the former chairman of the Democratic National Committee.

The new merged firm will go by the name Winston & Strawn and is one of the nation's 20 largest.

Winston & Strawn, concentrating on corporate and litigation work, will now have a presence in Chicago, New York and Washington, and it has its eye on Los Angeles as well.

Firm Chairman Gary L. Fairchild said the merger "gives us a real toehold in the Washington-based regulatory practice."

Also, he said, "To remain competitive as a national firm ... you have to continue to grow numerically as well as geographically."