The effect of transformation by Rous sarcoma virus on the neural cell adhesion molecule N-CAM was assessed by immunoblotting, immunofluorescence staining, and an in vitro cell-cell aggregation assay using highly specific antibodies to the adhesion molecule. Expression of N-CAM was found to be temperature dependent in several rat cerebellar cell lines infected with a mutant Rous sarcoma virus that is temperature sensitive for transformation. At the nonpermissive temperature, these cells displayed significant quantities of N-CAM and aggregated rapidly by an N-CAM-mediated mechanism. However, when the cell lines were grown at the permissive temperature, they were morphologically transformed, contained much lower amounts of N-CAM, and aggregated poorly. A similar temperature dependence of N-CAM expression was not observed in cultured primary rat cerebellar cells nor in a chemically transformed neuronal cell line. In all of the cell lines, N-CAM occurred in the adult forms; the embryonic form has so far been observed in normal embryonic tissues and a few regions of the adult brain. The findings show that N-CAM prevalence at the cell surface can be modulated by transformation with clear-cut effects on cell-cell adhesion.