In contrast to several previously published reports, we demonstrate by a variety of antibody assays that bovine serum albumin (BSA) is not antigenically cross-reactive with the 69-kDa islet cell autoantigen (ICA69). Fast protein liquid chromatography purified BSA and highly purified recombinant human ICA69 were used to establish sensitive Western blot and ELISA assays in order to detect antibodies against these two proteins. The assays excluded BSA or powdered milk as blocking agents, since these would interfere with antibody binding. A panel of sera from diabetic individuals, first degree relatives, and normal controls showed that the majority (approximately 70%) of individuals from each group had antibodies against ICA69 as assayed by Western blots, whereas considerably fewer (approximately 13%) had anti-BSA antibodies on Western blots, and individuals with antibodies to both proteins occurred only rarely (2-3%). To explore this issue further we immunized a total of 16 individual rats, representing four different strains (bio-breeding diabetes resistant and diabetes prone, Wistar-Furth, and Sprague-Dawley), with either BSA (n = 2 of each strain) or with recombinant ICA69 (n = 2 of each strain), and for each animal pre- and postimmune antibody titres against BSA and against ICA69 were assayed separately by enzyme-linked immunoabsorbent assay. In rats immunized with BSA, anti-BSA titres increased about 100,000-fold over preimmune levels, whereas anti-ICA69 reactive antibodies were unchanged over preimmune levels. Similarly, in rats immunized with ICA69, anti-ICA69 titres rose about 100,000-fold over preimmune levels, whereas anti-BSA antibodies were unchanged over preimmune levels. Thus we find no evidence for the existence of antibody cross-reactivity between ICA69 and BSA, either in humans or in immunized rats. When our rat anti-BSA antisera were used to probe Western blots made from rat islets isolated in the strict absence of fetal calf serum, we were unable to detect a 69-kDa band, even when the islets were preincubated with gamma-interferon, a treatment which has been reported to induce the BSA cross-reactive islet antigen. We conclude that BSA is not antigenically cross-reactive with ICA69 at the antibody level, and it is highly unlikely that BSA is antigenically cross-reactive with some other 69 kD islet cell antigen.