Regulation of steel factor (SF) production in Sertoli cells from postnatal mouse testes was studied in a mast cell-Sertoli cell coculture system. Treatment of Sertoli cells with dibutyryl cAMP (50-1000 mumol l-1), forskolin (1-25 mmol l-1), and cholera toxin (10 micrograms ml-1) increased SF production, whereas FSH and theophylline had no significant effect. Furthermore, growth factors and testosterone, which would play some roles in spermatogenesis, were also tested, but none of these stimulated SF production. The constitutive production of SF may, rather, reflect the physiological condition of Sertoli cells in vivo.