When 4 independently derived variant cell lines not expressing Thy-1 antigen were fused with parental cells expressing Thy-1, all fusion products expressed Thy-1 antigen. The fusion of variant lines with each other grouped these 4 recessive variants into 3 complementary classes. Variants which expressed a reduced quantity of Thy-1, antigen were isolated from the parental tumor cell population by an indirect selection method in which the variant cells were never directly exposed to antibody and complement. The experiments demonstrated that quantitative variants could occur spontaneously and did not require induction by the reagents used to select for them.