Patterns of phenotypic expression have been examined for two drug resistance markers originating by de novo variation in clonal lines of Chinese hamster cells. Sublines sensitive and resistant to cytosine arabinoside (ara-C) and vinblastine sulfate (VEL) were derived by selection from mass populations with the two inhibitors. Clonal derivatives of these lines were used to construct hybrids by spontaneous fusion in mixed cultures; the resulting hybrids were isolated in HAT medium without selective pressure on ara-C or VEL markers. Hybrids and parental lines were assayed by plating efficiency tests for resistance to VEL and ara-C. The results show that VEL resistance is expressed as a dominant or codominant characteristic, while resistance to ara-C is uniformly recessive. No cross-resistance or interaction between the two markers was observed. A variable factor in different lots of serum conditioned the toxicity of VEL; this effect was absent with ara-C.