The inhibins are α:β heterodimeric growth factors that are members of the transforming growth factor-β family. To understand the physiological roles of the inhibins in mammalian development and reproduction, a targeted deletion of the α-inhibin gene was generated by homologous recombination in mouse embryonic stem cells. Mice homozygous for the null allele (inhibin-deficient) initially develop normally but every mouse ultimately develops mixed or incompletely differentiated gonadal stromal tumours either unilaterally or bilaterally. Inhibin is thus a critical negative regulator of gonadal stromal cell proliferation and the first secreted protein identified to have tumour-suppressor activity.