Abstract Sixteen prepubertal Zebu crossbred bulls, aged 378 ± 4.7 days, weighing 189 ± 5.3 kg and with a scrotal circumference (SC) of 18.6 ± 0.6 cm were used to test the effect of active immunization against testosterone (T) on SC, liveweight change and luteinizing hormone (LH) and T secretion. Bulls were randomly allocated to receive one of four treatments: control (C), no treatment; immunization with 10 mg ovalbumin (OVA); immunization with 5 (T5); or, 10 (T10) mg of a T-OVA conjugate. Antibodies to T were titrated weekly, and expressed as percentage binding of added [ 3 H]T at a final serum dilution of 1:700. Concentrations of LH and T were measured in serum samples taken every 10 min for 6 h in Weeks 0, 8 and 12 of the study. Maximal titers obtained for Groups T5 and T10, between Weeks 11 and 12 were 76 and 57% (SEM = 4%), respectively. Bulls in Group T5 had greater SC than those in Groups C and OVA but only between Weeks 10 and 16 ( P −1 for bulls in all groups, but at Week 8, T was greater for bulls in Group T5 (9.68 ng ml −1 ; P −1 , compared with 4.9 ng ml −1 for Group T10 and less than 0.18 ng ml −1 for groups control and OVA ( P