We have analysed whether infiltration of macrophages and lymphocyte subtypes into pancreatic islets of diabetes prone BB rats occurs at random or whether insulitis requires a specific sequence of events. Serial sections from more than 700 islets of diabetes prone BB rats (70-150 days of age) were analysed for infiltrating immunocytes and expression of major histocompatibility complex antigens by 4-11 different monoclonal antibodies. In parallel, electron microscopy was performed in a fraction of islets. Part of the animals had been treated with macrophage toxic silica particles. A specific non-random sequence of events was identified and 4 stages of islet inflammation were recognised. Stages 1a and 1b are defined by macrophage (ED1+, W3/25+. Ox3/6/17+, ED2-) infiltration and concomitant enhanced major histocompatibility complex class I antigen expression initially at one pole or at the periphery of islets. T-, NK- and B-lymphocytes are absent (less than 1 cell per mean islet section). In stage 2, more macrophages are infiltrating and concomitantly Ox19+-T-lymphocytes and Ox8+-granular (NK-) lymphocytes are observed. In stage 3, additional massive infiltration of Ox12+-B-lymphocytes is noted. Silica treatment of BB rats largely prevented macrophage infiltration. Concomitantly islets were free of lymphocytes. Thus, macrophage infiltration clearly precedes T- and NK-lymphocyte and later B-lymphocyte infiltration. Lymphocytes do not infiltrate islets in the absence of prior macrophage invasion.