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 (
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70 % ) of individuals from each group had antibodies against ICA69 as assayed by Western blots, whereas considerably fewer (
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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 (
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