Anchorage-independent growth of tumor cells constitutes a phenotype highly associated with malignant transformation and appears to be important in the ultimate event of tumor metastasis, i.e., secondary tumor colonization. The role of a specific, melanoma-associated chondroitin sulfate proteoglycan population in anchorage-independent growth was assessed. Melanoma cells cultured in soft agar containing monoclonal antibody (mAb) 9.2.27, which recognizes such molecules on the surface of these cells, showed a 67-74% specific decrease in their colony formation. In contrast, neither mouse myeloma IgG nor monoclonal anti-HLA-A,B,C antibody (W6/32) had any effect on colony formation of the melanoma cells grown in soft agar. Human melanoma cells cultured in the presence of mAb 9.2.27 or W6/32 did not exhibit any changes in their DNA or protein synthetic metabolism. These findings suggest that 9.2.27-defined chondroitin sulfate proteoglycans on the surface of human melanoma cells may be involved in cell--cell interaction important in anchorage-independent growth.