Highly enriched populations of B lymphoblasts have been isolated after culture with anti-Ig-Sepharose and compared with dendritic cells as stimulators of CD4+ T cells in the murine MLR. The two populations clearly differed in phenotype; anti-Ig blasts were FcR+, B220+, 33D1-, while dendritic cells were FcR-, B220-, 33D1+. However, as MLR stimulators, they shared many common features. Both cells (a) expressed comparable levels of class II MHC products; (b) independently stimulated the primary MLR and the production of several T derived lymphokines including IL-2 and IL-4; and (c) were comparable in stimulating freshly sensitized T cells. However, the relative potencies of dendritic cells and anti-Ig blasts as primary MLR stimulators varied in a strain-dependent fashion. Only anti-Ig blasts could stimulate across an Mls barrier, being at least 100 times more active in stimulating Mls-mismatched, MHC-matched T cells, relative to syngeneic T cells. In contrast, dendritic cells were 10-30 times more potent than anti-Ig blasts when stimulating across an MHC barrier and were likewise more effective in binding MHC-disparate T cells to form the clusters in which the MLR was generated. Dendritic cell-T cell clustering was resistant to anti-LFA-1 mAb, while B blast-T cell clustering was totally blocked. Thus, anti-Ig B lymphoblasts and dendritic cells, two cell types which differ markedly in phenotype, also differ in efficiency and mechanism for initiating responses in allogeneic T cells.