A D.C. Superior Court judge yesterday warned prosecutors that he had "very serious reservations" about their efforts to subpoena a public defender to testify before a grand jury concerning a client he once represented.

The U.S. Attorney's Office is seeking the testimony of public defender James McComas in the criminal case involving Keith Lynn Roddy, who is charged with manslaughter in the starvation death of his infant son last January. Prosecutors want McComas, who represented Roddy on a rape charge in 1981, to testify concerning a conversation he had with Roddy at the Public Defender Service two to six weeks before the child died.

But Judge Henry F. Greene warned prosecutors that their subpoena of McComas is "rife with the possibility of undermining one of the most fundamental underpinnings that make the criminal justice system work--the attorney-client privilege."

Greene added that cases involving efforts to subpoena defense attorneys are rare.

He Greene postponed ruling on a request by Roddy's attorney to quash the subpoena until after both sides can present witnesses in the case on May 9.

Assistant U.S. Attorney F. Joseph Warin told the judge that the decision to subpoena McComas was "made at the highest levels of the U.S. Attorneys Office after much thought and research." He said that McComas' testimony is a "necessity" for the grand jury to complete its investigation.

Warin maintained that Roddy's conversation with McComas was not covered by the laws safeguarding attorney-client confidentiality because Roddy was not a client of the public defender service at the time the conversations in question took place. Beyond that, Warin said, the prosecutors had information that the conversations did not involve legal matters.

But Roddy's attorney Timothy D. Junkin accused prosecutors of "intruding into a person's ability to get legal counsel."

He said that Roddy was last represented by a member of the public defender service in July 1982 when he was placed on probation for a heroin conviction.

Junkin maintained that Roddy remained a client of the public defender service as long as he was on probation.

He said Roddy was still on probation at the time he spoke with McComas.

Junkin said that McComas will comply with the subpoena if ordered to do so by the judge.

Roddy, 29, was arrested Jan. 26 and has been charged with manslaughter after police found his 9-month-old son Keith dead of malnutrition in Roddy's Benning Road NE apartment.

Roddy told police that he had no money and could not get help from city agencies for his family.