Michael Ritz Jr., 51, a Hyattsville lawyer and former assistant corporation counsel in Washington, died of bone cancer Wednesday at the Washington Hospice.

He was admitted to the hospice on July 31 as the first patient at the new facility set up to care for the terminally ill. This area's first hospice, it is located in the Washington Home and is a joint venture of Georgetown University's Vincent T. Lombardi Cancer Research Center, Blue Cross and the Home.

Mr. Ritz had a general practice in criminal and civil law in Hyattsville since 1971. Before that he had a private law practice in Washington for 11 years.

He was born in Hazelton, Pa., and served in the Pacific with the Navy toward the end of World War II. He then earned a degree from Pennyslvania State University.

Mr. Ritz moved to Washington that year, and while working for the Democratic National Committee attended Georgetown University Law School, from which he graduated in 1953.

After working for three years as a legislative assistant to the late senator Estes Kefauver (D-Tenn.), he joined the D.C. Coporation Counsel's Office as an assistant, assigned to the Juvenile Court division.

He became an outspoken critic of the court's operations, and after a speech charging that it was in an "incredible mess" in 1960, Mr. Ritz resigned under pressure. Many community leaders agreed, however, that he had only voiced what many of them felt were problems at the court.

Mr. Ritz was a member of the D.C. and Maryland bars and the D.C. and Federal Bar associations. He belonged to the Father Rosensteel Council of the Knights of Columbus.

He is survived by his wife, Antoinette, and six children, Marcie, Suzie, Michael, Mary Ann, Julie and Cristie, all of the home in Silver Spring; his parents, Mr. and Mrs. Michael Ritz, of Allentown, Pa.; four brothers, Joseph, of Hazelton, Jack, of Hagerstown, Md., George, of Clarks Grove, Minn., and Ronald, of Allentown, and three sisters, Anna Barnhart, of Ridley Park, Pa., Helen Dargay, of Philadelphia, and Marie Thatcher, of Rockville.

The family suggests that expressions of sympathy be in the form of contributions to the Washington Hospice.