An assistant to a Manassas doctor who lost his medical license for injecting an unapproved aloe vera drug into cancer patients has agreed to testify against his former employer, prosecutors said.

Ronald R. Sheetz, a former auto mechanic with no formal medical training, pleaded guilty last week to two felony counts of prescription fraud. Four felony charges of false pretenses were reduced to misdemeanors as part of his agreement. Prince William Commonwealth's Attorney Paul B. Ebert said that Sheetz agreed to help prosecutors in their case against Donald L. MacNay, an orthopedic surgeon charged with defrauding the cancer patients.

Sheetz, 42, of Manassas, "agreed to cooperate with the commonwealth," Ebert said. "He is a potential witness in the upcoming case."

Reached at his home, Sheetz declined comment.

MacNay came under scrutiny in September 1997, when two men died shortly after receiving an aloe-vera-based treatment at his clinic.The men, one from Michigan and another from Texas, had traveled to Manassas in the hope of finding a miracle cure and died in MacNay's office shortly after treatment began.

Investigators subsequently linked two other deaths to MacNay's treatments. The victims' relatives told authorities that MacNay lied about the alleged benefits of the treatments and bilked them of thousands of dollars. They also told officials that MacNay never reviewed the patients' medical histories or conducted physical exams before starting treatment.

The Virginia Board of Medicine voted unanimously to strip MacNay of his medical license in March 1998, citing fraud, unprofessional conduct and gross malpractice. In April 1998, a Prince William County grand jury indicted MacNay on two counts each of false pretenses, attempted false pretenses and supplying the painkillers Dilaudid and morphine to Sheetz. His trial is scheduled for September.

In Maryland, the attorney general's office launched an investigation into the Baltimore company that allegedly supplied MacNay with the aloe vera substance. Maryland officials allege that Allen Hoffman, owner of T-UP Inc., and associate Neal Deoul violated state consumer protection laws by making false claims about the safety and effectiveness of their product, known as T-UP, and lying about the research and medical qualifications of the people who sell it.

Sheetz, who entered his plea in Prince William County Circuit Court, is scheduled to be sentenced Aug. 12. Prosecutors have recommended probation. Circuit Court Judge Herman A. Whisenant could sentence Sheetz to up to five years in prison.