In this study, subsets of mononuclear infiltrates in pancreatic islets of BB rats at different stages of insulitis were determined by various monoclonal antibodies against rat lymphoid cells, including a mouse monoclonal IgM antibody that distinguishes between macrophages and dendritic cells (mAb 1F119). 1F119+ dendritic cells were absent in and around islets of Wistar control rats. In BB rats, the first alteration of islets detectable by immunohistochemistry when compared with normal islets was the enhanced expression of 1F119 antigen around and in the islets (17% 1F119+ islets). At disease stage 1 (i.e. no leukocyte infiltration after HE staining), lymphocytes and macrophages were almost absent. At disease stage 2 (leukocyte infiltration < 20 cells), a more intense form of dendritic cell infiltration was seen (stage 1 versus stage 2, P < 0.0001). In addition, ED2+ and ED3+ cells were present around the islets (50% of islets were infiltrated with 1F119+ cells versus 16% with ED2+, 19% with ED3+, 11% with W3/25+ cells, and 11% with OX8+ cells, P < 0.0001). At disease stage 3 (> 20 cells), a clear increase of ED2+ and ED3+ macrophages and of W3/25+ and OX8+ T-lymphocytes in the infiltrates was observed. These observations suggest a role for antigen presenting dendritic cells in the initiation of immune reaction in type 1 diabetes of BB rats.