We examined the effect of laminin on DNA synthesis or cell growth of bone marrow-derived macrophages (BMM) and five macrophage cell lines including two factor-dependent macrophage cell lines (BDM-1 and BDM-1W3) and three factor-independent ones (BAM3, J774.1, and P388D1). Laminin stimulated the M-CSF-dependent proliferation of BMM and BDM-1 cells in a dose-dependent manner, but it had no effect on the proliferation of BDM-1W3 cells. This stimulatory effect was modified by increasing the concentration of fetal calf serum in the culture medium. Laminin also stimulated the proliferation of BAM3 and J774.1 cells, whereas it could not stimulate that of P388D1 cells. Taken together, these results suggest that the differences in the response of these macrophage cell lines to the presence of laminin may be derived from the difference in the state of maturation of these cells. Although laminin acted as a potent mitogenic factor when it was added to the culture medium, the stimulation was two- to threefold lower when dishes were coated with equivalent amounts of laminin. When BDM-1 cells were cultured with laminin, they underwent drastic morphological changes. A monoclonal antibody that recognizes the integrin alpha 6 subunit only partially inhibited the mitogenic effect of laminin. The results in this report suggest that laminin may participate in the regulation of macrophage populations in vivo.