When viral envelope antigens (VEAs) and virus-associated cell surface antigens (CSAs) are used as markers of type C viruses, more heterogeneity of virus populations can be demonstrated than by using host range alone. In the present study, four CSAs (PC1, X.1, GCSA, and MEV-SA1) and three VEAs (xVEA, x1-VEA, and sub-gsVEA) were used as markers of virus populations present in various sublines of the BALB/c embryonic fibroblast BALB/3T3 clone A31 line. Of four spontaneously transformed sublines, two released N-tropic endogenous type C viruses spontaneously after long-term culture and each had distinct antigenic patterns. Treatment with 5-bromodoxyuridine (BrdU) resulted in the detection of X-tropic viruses in all four sublines. The expression of these X-tropic viruses was associated with two different antigenic patterns. When the clonal rabbit corneal SIRC cell line was infected with the X-tropic viruses obtained after BrdU treatment, the cells acquired detectable amounts of only one of the antigenic markers.