Sera from diabetes-prone BB/OK rats were tested for humoral-mediated cytotoxicity to rat pancreatic islet cells, spleen lymphocytes and exocrine pancreatic cells using 51Cr-release assay systems. At the age of 20, 30 and 40 days all cross-sectionally studied BB/OK rats showed cytotoxicity to islet cells but only 37.5%, 25.0% and 63.3% of them exhibited anti-lymphocyte cytotoxicity, respectively. Neither the time course of cytotoxicity to islet cells nor to lymphocytes differed in BB/OK rats developing diabetes compared to animals maintaining normoglycaemia as evidenced in a follow-up study. The decrease of cytotoxicity to islet cells in vitro as observed in the time course study seems to be due to the appearance of an inhibitor of anti-islet cell cytotoxicity in serum from BB/OK rats older than 70 days. However, under conditions avoiding the influence of inhibitory components the observed time course of anti-islet cell cytotoxicity also did not permit to distinguish potential diabetic BB/OK rats from animals maintaining normoglycaemia. In contrast, long-term normoglycaemic BB/OK rats showed a peak value of cytotoxicity to rat exocrine pancreatic cells between 40 and 50 days of age only whereas animals developing diabetes more frequently displayed cytotoxicity in the prediabetic phase. Inhibitory activity against cytotoxicity to exocrine cells was not likewise detectable in BB/OK rat serum. In conclusion, except of more frequently appearing cytotoxicity to rat exocrine pancreatic cells among the investigated BB/OK rats becoming diabetic the cytotoxicity patterns to islet cells and spleen lymphocytes were not predictive for diabetes onset. Thus, humoral-mediated cytotoxicity seems to appear in BB/OK rats as a sign of immune dysregulation characteristic for this animal model at high risk for diabetes rather than in a manner related to disease manifestation.