The presence of endogenous, galactoside-specific lectin molecules on the surface of various neoplastic cells has been demonstrated recently using monoclonal antibody (mAb) 5D7 [Raz et al., EMBO (Eur. Mol. Biol. Organ.) J., 3: 2979, 1984]. The effect of this mAb on the growth of several transformed and tumor cell lines of murine and human origin was investigated using in vitro techniques. A dose-dependent reduction (30 to 100%) in colony formation on a solid substrate or in a semisolid medium was observed when the cells were cultured in the presence of 15 to 100 micrograms of mAb 5D7 per ml of medium. Inhibition of anchorage-independent growth was more pronounced (2- to 3-fold) than inhibition of anchorage-dependent growth for most of the cells. The growth-inhibitory effects of mAb 5D7 were not the result of a cytolytic activity, for neither DNA nor protein synthesis was suppressed in semiconfluent cell cultures after 3 days of exposure to the antibody. Other mAbs that recognize cell surface components, such as chondroitin sulfate or fibronectin, failed to inhibit colony formation. These results suggest that endogenous tumor cell-surface lectin molecules may be involved in intercellular interactions or interactions between the cells and exogenous ligands; these interactions are important for growth regulation.