To the Editor: A number of drugs have been shown to interfere with cyclosporine metabolism, by altering either enterohepatic transport or hepatic metabolism. Recently, we studied an interaction between cyclosporine and erythromycin.The patient was a 56-year-old white man who received a renal transplant from a living unrelated donor in July 1984 for end-stage renal disease due to hypertensive nephrosclerosis. After transplantation, he had a stable creatinine level of 1.8 mg per deciliter while taking 15 mg of prednisone daily and cyclosporine (while taking 325 mg daily over a period of six months, his trough whole-blood levels of cyclosporine according . . .