Complement-fixing antibodies were found from rabbit sensitized by simple injection of Insulin (fish Insulin & animal Insulin), without hitherto-used horse serum etc. 1) By the cross reaction between fish Insulin and animal Insulin, Insulin complement-fixing antibodies were found after one hour of fixing antigen, antibodies and complement at 37℃. and prolonged 24 hours at 3℃; which were never found by Kolmer method. Insulin, as its molecular weight is small, takes a long time to react Insulin complement-fixing just as the case of Virus. 2) Insulin sensitized serum is positive, when complement-fixing reaction is made using extracts of fish and beef as antigen. This shows that the serum contains foreign matters besides Insulin antigen. 3) The same results were obtained by using crystal Insulin as well as commercial Insulin. 4) Insulin complement fixing antibodies last 3 or 6 months, and strongest reactions are found 7 days after the sensitization. 5) Complement-fixing antibodies were found by a cross reaction rather than by serum of patients treated by Insulin.