A New York doctor pleaded guilty in U.S. District Court here yesterday to conspiring to illegally distribute the diet drug, Preludin and the pain killer Dilaudid and now is expected to be a major government witness against the alleged D.C. kingpin of the pill-selling operation.

Dr. Alfred Calfon told Judge Thomas A. Flannery that he wrote more than 300 prescriptions for the two drugs for phantom patients between the end of 1975 and June 1977. For his efforts he said he was paid $16,000 during that time by Carl L. (Cobby) Lynch, a Hillcrest Heights man and the accused mastermind of the drug scheme, and Lynch's operatives.

Calfon, 48, said that he got $25 for a Preludin prescription and $50 for a Dilaudid prescription. Dilaudid is the more potent of the two drugs and often is called "hospital heroin" because it is legitimately given people after operations and terminal cancer patients as a pain killer.

Calfon was the sixth person to plead guilty in the drug operation. The government charged that Lynch and his lieutenants, many of them his relatives recruited 15 to 18 women, most of them poor and fat, to make trips to various cities, including New York, where Calfon and others wrote prescriptions for the pills. The pills were then allegedly sold at inflated prices to drug addicts on the streets of Washington.

Calfon, who is a naturalized U.S. citizen and a graduate of the University of Paris medical school, faces up to 15 years in jail and a $25,000 fine. However, the prosecutors in the case, assistant U.S. attorneys Charles J. Harkins and Peter O. Mueller, promised as part of the pea bargaining arrangement that they would inform Flannery of any help Calfon gives the government in the case as a mitigating factor in his sentence.

A Philadelphia doctor, Lee D. Hedson, 64, pleaded guilty last month to conspiring to distribute Dilaudid and faces up to five years' imprisonment and a $15,000 fine. When he was indicted in August, Hedson was the medical director of the Pennhurst Center, a state-run hospital in Spring City, Pa.

Irvin Wood Jr., 36, of 1506 Iverson St., Oxon Hill: Theresa E. Farrell of 912 9th St. N.W. J. Lynch, 29, of 2424 St. Clair Dr., Hillcrest Heights, a half-brother of Carl Lynch, and James Ferguson, 39, of 1621 Ft. DuPont St. SE, Have also pleaded guilty to conspiring to distribute Dilaudid, according to the prosecutors.

Carl Lynch's trial is scheduled to start Oct. 30.