Summary. Groups of bull calves received a primary immunization against testosterone (Group T; N = 7) or oestradiol-17β (Group E; N = 9) at 3 months of age and booster injections on four occasions at ∼2 month intervals. Controls (Group C, N = 7) were immunized against human serum albumin alone using the same protocol. Immunity was achieved against both steroids as judged by the secondary antisteroid antibody titres in Group T (730 ± 231; reciprocal of titre) and Group E (12 205 ± 4366) bulls; however, peak antibody titres generally declined with successive booster injections. Mean plasma concentrations of LH, FSH and testosterone during the period from 3 to 10 months of age were higher (P < 0·05) in Group T bulls than in Groups C and E. Group T bulls had larger testes compared with controls from 6 months of age onwards. At castration at 14 months of age, testes of Group T bulls were heavier (P < 0·05) than those of Groups C and E (179 ± 13, 145 ± 8 and 147 ± 6 g, respectively). At 10 months of age, there were no differences among treatment groups in LH responses to LHRH, but the testosterone responses were greater (P < 0·05) in bulls in Group T (26·2 ± 4·9 ng/ml) and Group E (16·6 ± 1·8 ng/ml) compared with those in Group C (6·9 ± 0·6 ng/ml). Testosterone responses to hCG determined at 13 months of age were also greater (P < 0·05) in Groups T and E relative to controls. At 14 months of age daily sperm production rates per bull (× 10−9) were higher (P < 0·10) in Group T bulls (2·2 ± 0·1) than those in Groups C (1·6±0·2) and E (1·6±0·1). These results indicate that early immunity against testosterone is associated with increased gonadotrophin secretion and accelerated growth of the testes in prepubertal bulls. Also, chronic immunity against testosterone or oestradiol-17β enhances the steroidogenic response of bull testes to gonadotrophic stimulation. If the above responses observed in young bulls are shown to be sustained, then immunity against gonadal steroids early in life may confer some reproductive advantage in mature animals.