A federal judge in Alexandria yesterday repeatedly questioned the motives and ethics of two doctors who are seeking a court order to enable them to return to practice at a private Northern Virginia kidney dialysis clinic that now bans their presence.

"Don't tell me you're doing this for humanitarian reasons," U.S. District Court Judge Oren R. Lewis said to Dr. Robert E. Greenspan, one of the physicians. "You're doing this for money, big money, treble damages."

Greenspan and Dr. Steven Tolkan filed suit last month seeking $600,000 each in lost salary from the Northern Virginia Dialysis Center, its director, Dr. Raphael J. Osheroff, and the center's two Boston-based corporate owners.

The doctors claim that Osheroff and the other defendants have illegally kept them from treating their patients at the center center late last year. Four patients have also joined the suit, seeking the return of Greenspan and Tolkan to the center located at 5249 Duke St., Alexandria.

Yesterday Lewis zeroed in on the financial aspects of the kidney dialysis treatments, which cost $23,000 annually to keep a patient's blood free of poison.

Greenspan testified that last year he asked patients while they were hooked up to the dialysis machine if they wanted to sign a form stating they would have the Osheroff clinic and go to a clinic he would soon open. Lewis then said to him, "that's the first time I ever heard of the bartening of patients . . . did you consider that to be ethical, when they were . . . watching their own blood go up and down" in the dialysis machine?

Greenspan said it was ethical conduct, since he was giving the patients a choice of following him. Lewis called it a "Hobson's choice," meaning no choice at all.

Defense attorneys said outside of court they will argue in closing arguments today that Greenspan and Tolkan were attempting to take Osheroff's business from him, and Osheroff refused to let them return to the clinic when he found out about it.

Greenspan is trying to open a kidney dialysis center in Montgomery County, where his efforts are being opposed by National Medical Care Inc., one of the corporate defendants which owns 106 kidney clinics nation-wide, including the Alexandria clinic, and 10 of the remaining 14 in the metropolitan area.