In order to gain more insight into the pathogenetic mechanism leading to beta cell destruction in BB rats we searched for specific changes in the immune system at the time of diabetes development. We performed a density gradient fractionation of all major leukocyte types in the peripheral blood and the spleen of acutely diabetic or non-diabetic BB rats as well as of normal Wistar rats in order to identify proliferating low density blasts. Acutely diabetic BB rats showed a 4-6 times higher percentage of lymphoblasts among spleen cells than non-diabetic BB or Wistar rats. The majority of blasts was of the T-cytotoxic/suppressor (Ox8+) phenotype (61%) in acutely diabetic BB rats, but of the T-helper (W3/25+) phenotype (51%) in non-diabetic BB or Wistar rats. Plasma cell and monocyte numbers were not increased in acutely diabetic BB rats. At the time of diabetes manifestation BB rats develop a pronounced eosinophilia. These observations indicate a state of enhanced cellular immunity involving cytotoxic/suppressor T cells during diabetes development in BB rats.