A correlation between MUC1 expression in renal cell carcinomas (RCCs) and the clinical stages was previously demonstrated. To assess whether MUC1 expression is causally related to malignant tumor behavior, MUC1 cDNA was stably transfected into a renal carcinoma cell line SN12C that expresses trace levels of MUC1. MUC1 with sialylated carbohydrate chains was detected on the surface of transfected cells in two independent experiments. There was no correlation between MUC1 expression and in vitro growth and motility. In vivo growth of the transfectants at the site of orthotopic transplantation in nude mice was slower than mock transfected cells. Therefore, MUC1 alone did not seem to confer a malignant phenotype to RCCs.