In an attempt to characterize insulitis induced by multiple low doses of streptozocin (SZ), we immunohistochemically examined sequential changes of the subsets of lymphocytes infiltrating the pancreatic islets in CD-1 mice. Daily intraperitoneal injections of 30 mg/kg body wt of SZ for five consecutive days led to lymphocytic infiltration around the islets. Most of the infiltrated cells were initially CD4 positive (helper/inducer) T-lymphocytes and no immunoglobulin-bearing cells were detected. The number of helper/inducer T-lymphocytes increased with progression of the insulitis, then surface immunoglobulin-positive cells (B-lymphocytes) accumulated around the area of CD4 positive T-lymphocytes. CD8 positive (suppressor/cytotoxic) T-lymphocytes were seen scattered throughout the study and only a few asialo GM1 positive (natural killer) cells existed in the area of insulitis. Subsequently, the pancreatic insulin contents were considerably decreased and diabetes occurred on day 21. These observations suggest that CD4 positive T-lymphocyte-dependent B-lymphocyte infiltration in and around islet cells may be associated with islet cell destruction and the development of diabetes in CD-1 mice treated with multiple low doses of streptozocin.