The immunogenicity of biosynthetic human insulin (BHI) was studied in diabetic patients who had never received insulin treatment (Study A) and in diabetic patients who had already been treated with monocomponent insulin (Study B). The results of both studies were compared to matched control groups receiving other forms of insulin treatment. Blood samples obtained were tested for anti-insulin antibodies and circulating immune complexes using two different methods. After six months of treatment, the values of anti-insulin antibodies in those patients in Study A who were treated with BHI were significantly lower than those observed in control patients treated with monocomponent (P less than 0.02) or conventional insulin (P less than 0.001). At the sixth month of Study A no significant difference in the percentage of circulating immune complex positivity was seen between the three groups. In Study B no significant difference in the values of insulin antibodies or immune complexes was observed between patients who were switched to BHI and those who continued monocomponent insulin. No side effects were observed. The data show that the immunogenicity of BHI is even lower than that of monocomponent insulin.