The genetic terminology of sex determination and sex differentiation is examined in relation to its underlying biological basis. On the assumption that the function of the testis is to produce hormones and spermatozoa, the hypothesis of a single Y-chromosomal testis-determining gene with a dominant effect is shown to run counter to the following observed facts: a lowering in testosterone levels and an increase in the incidence of undescended testes, in addition to sterility, in males with multiple X chromosomes; abnormalities of the testes in autosomal trisomies; phenotypic abnormalities of XX males apparently increasing with decreasing amounts of Y-chromosomal material; the occurrence of patients with gonadal dysgenesis and XY males with ambiguous genitalia in the same sibship; the occurrence of identical SRY mutations in patients with gonadal dysgenesis and fertile males in the same pedigree; and the development of XY female and hermaphrodite mice having the same genetic constitution. The role of X inactivation in the production of males, females and hermaphrodites in T(X;16)16H mice has previously been suggested but not unequivocally demonstrated; moreover, X inactivation cannot account for the observed bilateral asymmetry of gonadal differentiation in XY hermaphrodites in humans and mice. There is evidence for a delay in development of the supporting cells in XY mice with ovarian formation. Once testicular differentiation and male hormone secretion have begun, other Y-chromosomal genes are required to maintain spermatogenesis and to complete spermiogenesis, but these genes do not function effectively in the presence of more than one X chromosome. The impairment of spermatogenesis by many other chromosome abnormalities seems to be more severe than that of oogenesis. It is concluded that the notion of a single testis-determining gene being responsible for male sex differentiation lacks biological validity, and that the genotype of a functional, i.e. fertile, male differs from that of a functional female by the presence of multiple Y-chromosomal genes in association with but a single X chromosome. Male sex differentiation in XY individuals can be further impaired by a euploid, but inappropriate, genetic background. The genes involved in testis development may function as growth regulators in the tissues in which they are active.