MUC18/MCAM is a cell-surface glycoprotein that is strongly expressed on advanced human melanomas. Transfection of 3 MCAM-negative melanoma cell lines with MCAM cDNA led to cell-surface expression and to a MCAM-dependent homotypic adhesion. This adhesion was independent of divalent cations and was inhibited at 4 degrees C. Mixed aggregation assays with MCAM-expressing and non-expressing cells revealed that MCAM can function as a heterophilic cell adhesion molecule interacting with a non-MCAM ligand. Although MCAM contains a potential glycosaminoglycan-binding site, cell-surface glycosaminoglycans do not appear to be involved in the heterophilic adhesion observed here since these molecules were not able to influence the adhesion. Using a functional adhesion assay, 4/4 melanoma cell lines examined were found to express an MCAM ligand. In contrast, no evidence for an MCAM ligand was found on the 2 carcinoma or 2 hematopoietic cell lines examined. Stable transfection of an MCAM ligand-negative colorectal cell line resulted in MCAM surface expression but not in homotypic adhesion, indicating that homophilic MCAM-MCAM adhesive interactions may not occur. Our results suggest that MCAM expression by melanoma cells is associated with increased homotypic adhesion, an event that may support tumor cell survival and growth in vivo.