Lectins purified by affinity chromatography on immobilized asialofetuin from extracts of mouse K-1735P melanoma cells appeared as two polypeptides [L-14.5 (Mr 14,500) and L-34 (Mr 34,000)] in one-dimensional polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. However, in two-dimensional electrophoresis (isoelectric focusing followed by sodium dodecyl sulfate:polyacrylamide gel electrophoresis) the L-14.5 polypeptide was resolved into three acidic forms of pI 4.6, 4.9, and 5.8, whereas the L-34 was resolved into two polypeptides of pI 4.9 and 5.3. Antibodies directed against galactoside-binding lectins from rat and bovine lungs, mouse 3T3 fibroblasts, and mouse UV-2237 fibrosarcoma cells reacted with the K-1735P lectins in immunoblots, and normal mouse lung extracts were found to contain cross-reactive proteins that comigrated with the two melanoma lectins. Indirect immunofluorescence staining using the above antibodies demonstrated that both L-14.5 and L-34 were expressed on the surface of viable K-1735P cells. Treatment of these cells with 1 microM beta-all-trans-retinoic acid or 1 mM N6,O2'-dibutyryl cyclic AMP for 5 days induced morphological differentiation, inhibition of anchorage-dependent and anchorage-independent growths, and a selective decrease in the L-34 lectin level. Growth inhibition by starvation for serum factors, which did not induce differentiation, had no effect on the level of L-34. These results demonstrate that the melanoma lectins are immunologically related to normal cell lectins and that the two polypeptide species are expressed on the cell surface. Further, they demonstrate that the L-34 lectin level can be modulated by agents that suppress the transformed phenotype by enhancing differentiation.