We have previously reported that 40% of diabetic patients have an impaired specific immune response after vaccination against B-hepatitis. Thymopentin (TP5), the active site of thymopoietin hormone, has been shown to increase antibody response (HbsAb) following B-hepatitis vaccination in several disease conditions. In the present study TP5 (50 mg) was administered subcutaneously three times per week to 17 diabetic patients for a week prior B-hepatitis vaccination and for three weeks afterwards. Sero-conversion was observed after the third dose of vaccine in 15 out of 17 (88%) patients and in 94% of a group of normal subjects acting as control. Although the median HbsAb titre was significantly lower in diabetics compared to normal subjects, we conclude that administration of TP5 in diabetic patients increases the rate of sero-conversion following B-hepatitis vaccination.