Summary.The disappearance rate of insulin antibodies was studied after cessation of insulin treatment which had been given for 3 months to 6 years in 42 Type 2 (non-insulin-dependent) diabetic patients. Insulin antibodies were measured before and 15 days after interruption of insulin treatment, and every 30 days until the disappearance of insulin antibodies. The mean + SD value of insulin binding in the entire group before the interruption of insulin treatment was 32 + 14%. There was no relationship between the antibody level at that time and the duration of insulin treatment. However, the insulin antibody level was significantly higher in 17 diabetic patients on an insulin dose of > 20 U/day (p < 0.02) than in 25 on an insulin dose of < 20 U/day (39 _+ 13% versus 28 _+ 12%). A positive correlation was found between intial insulin binding and the time required for it to fall below 10% (r = 0.74). Antibodies were absent 60 days after discontinuing insulin treatment in eight of ten subjects presenting with initial binding of < 20%. In contrast, in only two of 12 patients with an initial binding of > 40% were insulin antibodies detectable 150 days after discontinuation of insulin therapy. Disappearance of insulin antibodies sometimes took up to 1 year and occasionally even more than 2 years.