Mouse L cells, which do not express the known primary cell adhesion molecules (CAMs), were permanently transfected with vectors containing the simian virus 40 early promoter and cDNA sequences encoding chicken liver CAM (L-CAM) or each of the three major polypeptide forms of chicken neural CAM (N-CAM). Transfected cells in culture expressing the Ca2+-dependent L-CAM showed uniform surface expression of the molecule. Unlike untransfected L cells, these cells aggregated readily; the aggregation was inhibited by Fab' fragments of antibodies to L-CAM but not by fragments of anti-N-CAM. These cells spread more efficiently in culture than did their untransfected counterparts, forming small colonies of flattened cells that gradually assumed morphologies resembling closely packed L cells. Transfected L cells expressing either the small or large intercellular domain polypeptide (sd or ld) chains of N-CAM aggregated specifically with each other or bound membrane vesicles from chick brain. Both types of binding were specifically inhibited by Fab' fragments of anti-N-CAM antibodies. These cells, in contrast to those transfected with vectors for L-CAM, showed rounded morphologies and spread inefficiently in culture. L cells transfected with vectors specifying the small surface domain polypeptide (ssd) chain of N-CAM showed no phenotypic changes and no evidence for linkage of ssd chains to the cell membrane by phosphatidylinositol intermediates. Instead, these cells synthesized the molecule and released it into the medium. These findings complete the demonstration that different CAMs have specific roles in ligating the cells that synthesize them, and they provide further evidence that L-CAM and N-CAM bind by homophilic mechanisms. The different phenotypic changes observed for each specific CAM are consistent with the hypothesis that CAM synthesis or differing associations of CAM carboxyl-terminal domains with the cell surface and cortex may lead directly or indirectly to specific alterations in the cells bound together by that CAM.