The BB rat is a widely used animal model for the study of insulin dependent diabetes. An enzyme linked immunosorbent assay, using purified rat insulin, was used to measure serial insulin autoantibodies (IAA) levels in coded sera from the BB/W/D rat colony in order to establish the time course of IAA. The animals included 26 diabetes-prone BB rats, six diabetes-resistant BB rats and six Wistar controls. There was an increase in both IAA frequency and titre with time in the diabetes-prone group: none were positive at 45 days, 17/19 (89%) were positive by day 90 and all were positive thereafter. Similar results were observed in the diabetes-resistant BB group (0/6 positive at day 51, 6/6 positive at day 90). None of the Wistar controls were positive at 105 days, although occasional positive sera were observed at 120 days. IAA seem to be acquired early on in the majority of BB rats, both diabetes-prone and diabetes-resistant, and much later, if at all, in controls. A clear homology of the MHC genes exists in both BB rat sublines, thus IAA appear to be a strain related phenomenon rather than a marker for IDDM.