The effect of cholera toxin (CT) on the thyroid-pituitary axis and the immune system was examined in Bio-Breeding/Tokyo (BB/TKY) rats, which spontaneously develop insulin-dependent diabetes mellitus (DM) and lymphocytic thyroiditis (LT). Intravenous administration of CT (5 micrograms/100 g body weight) every other week starting at 6 weeks of age resulted in a significant decrease in the serum thyrotropin (TSH) level at 12 and 14 weeks of age when compared with saline treated littermates. CT stimulated rat thyroid cells to proliferate in vitro. Furthermore, serum anti-thyroglobulin antibody (ATA) titers were also significantly decreased in 14-week-old rats treated with CT. In vitro ATA production by spleen cells from BB/TKY rats was inhibited by CT. Antibodies to thyroxine were detected in both CT-treated and control rats. It is of interest that the ratio of W3/25+ helper/inducer cells to OX8+ suppressor/cytotoxic cells was significantly decreased in CT-treated rats. However, there was no significant difference in the incidence of DM and LT between the two groups of rats. The present study showed that CT suppressed ATA production both in vivo and in vitro, and had a stimulatory effect on thyrocytes in BB/TKY rats.