To evaluate whether pregnancy has any effect on insulin antibody levels and to test the concordance between a conventional radioimmunoassay and a new microassay for the detection of insulin antibodies, insulin antibodies were analysed in 104 mothers in early pregnancy and at delivery and in their newborn infants. Thirty-eight of the mothers had type 1 diabetes. The concordance between the assays was high in the samples taken in early pregnancy (95%), but substantially lower in the samples taken at delivery (40%) and in the cord blood samples (68%). A considerable proportion of the mothers at delivery, especially the unaffected mothers (71%), and the newborn infants of the unaffected mothers (32%) were positive for insulin antibodies in the conventional assay but not in the microassay. Insulin antibody levels increased in the mothers, significantly so in the unaffected mothers (P < 0.001), during pregnancy in the conventional assay, whereas in the microassay they decreased significantly (P < 0.01) in affected mothers and remained negative in the unaffected mothers. Since immune complexes are precipitated with protein A specific for IgG in the microassay and with polyethylene glycol lacking specificity for immunoglobulins in the conventional assay, our data indicate that insulin antibody levels decrease on average during pregnancy and that the increasing non-IgG anti-insulin activity observed in the conventional assay is induced by pregnancy and is present in both the maternal and the foetal circulation.