RNA from immature mouse testes was shown to lack a low-molecular-weight c-abl transcript previously noted to be the predominant species in adult testes. The developmental pattern of appearance of this c-abl variant was determined by analyzing RNA obtained from purified populations of testicular cells in different stages of spermatogenesis. The appearance of the c-abl testicular variant was coincident with the entry of the germ cells into their haploid state and suggested that the regulated expression of this proto-oncogene may be important in the normal differentiation of the male germ line.