Cloned BALB/c 3T3 cells transformed in vitro with polyoma virus (PyV) acquired a higher tumorigenicity phenotype after a single in vivo passage. Some of the in vivo passaged cells (CTC cells) exhibited also a higher metastatic phenotype than cells from the same clones that were maintained only in culture (C cells). A phenotypic comparison between CTC and C cells was performed. It was found that most CTC lines exhibited a higher binding to laminin compared to their clonal C cell ancestors. Some CTC cells were less sensitive to the cytotoxic effects of TNF-α than the corresponding C cells. CTC cells originating from tumors which appeared after a long latency period (late tumors) tended to express FcγRII while CTC cells originating from tumors which appeared after a short latency period (early tumors) as well as the corresponding C cells tended not to express FcγRII. The expression of a membrane epitope recognized by a monoclonal antibody expressing specificity towards PyV transformed cells, was down-regulated on late tumor cells compared to early tumor cells. Transfection of cloned PyV-transformed BALB/c 3T3 cells with the β1FcγRII gene augmented the tumorigenicity and metastatic phenotype of the transfectants compared to control transfectants.