RSV-transformed rat sarcoma (XC) cells were characterized as to their ability to anchorage-dependent and anchorage-independent growth in various culture conditions. The proliferation of XC cells was dependent on serum concentration and initial cell density. The relationship between seeding density and the growth rate of XC cells indicated that the investigated cells produce factors with growth stimulating and growth inhibiting activity. The anchorage-independent growth inhibiting activity was found in ultrafiltrate (500 Mr 10,000) of XC cells conditioned medium. The obtained results suggests that anchorage-independent growth of XC cells may be regulated by two types of autocrine factors, which are antagonistic in their biological affects.