Abstract Splenocyte proliferation as measured by [3H]thymidine incorporation was detected when brain microvessel smooth muscle cells (SM) were cocultured with syngeneic spleen cells. This report focuses on the role of different lymphocyte populations in this activation. The central role of CD4+ T cells in the proliferation response has been established by different sets of experiments. The phenotypic characterization of splenic lymphocytes before and after the co-culture showed that the only cell type present in higher number after the co-culture than before is the CD4+ T cell. When CD4+ cells were purified by flow microfluorimetry and co-cultured with SM a strong proliferative response was detected. In contrast, purified CD8+ cells in co-culture with SM cells did not proliferate. The activation of CD4+ cells by SM required direct cell-to-cell contact and could be detected on the fourth day, reaching maximal levels at the 6th and 7th days of the co-culture. The activation is more pronounced in the syngeneic system than under allogeneic conditions and is inhibited by anti-MHC II mAb, but not by anti-MHC II mAb. The finding that vascular smooth muscle cells can activate syngeneic T cells may have important implications concerning the mechanism of induction of vasculitis.