Chloramphenicol resistance, a dominant marker, is readily transferred between Chinese hamster cells by fusing enucleated cytoplasts from resistant cells to a sensitive recipient strain. Isolation of cytohybrids depends on complementary selection for recessive characteristics present in recipient cells, e.g. resistance to bromodeoxyuridine. Sources of background variation in this system have been examined to determine the efficiency of cytoplasmic marker transfer. The frequency of mutants resistant to both chloramphenicol and bromodeoxyuridine was minimal in the parent strains used. Dual resistance was found to arise, however, by segregation within hybrids formed between nucleated cells present as contaminants in cytoplast fusion mixtures. Background variation may also simulate cytoplasmic transfer of nuclear markers from donor cells (e.g. resistance to ouabain). Cytohybrids resistant to both ouabain and chloramphenicol were in fact isolated from cytoplast fusion mixtures by selection with ouabain and bromodeoxyuridine. Such variants appeared to originate by mutation to ouabain resistance in recipient cells followed by cytoplasmic transfer of the chloramphenicol resistance marker.