The aim of the present study was to evaluate the effects of gonadotropin administration at initiation of inhibin passive immunisation in Jersey bull calves (age 27 ± 5 days) on testicular morphology and development. Primary treatments consisted of control (keyhole limpet haemocyanin, KLH; n = 9) or immunisation against inhibin (INH; n = 9). Subsets of calves were randomly assigned within primary treatments (TRT) to receive saline ( n = 3 per TRT), follicle-stimulating hormone (FSH; n = 3 per TRT) or gonadotrophin-releasing hormone (GnRH, n = 3 per TRT). The right testis was removed (age 118 ± 5 days) to determine volumes of testicular components and cell numbers per testis using stereology. Data were analysed using the MIXED procedure of the SAS program. Antibody titres against inhibin were increased in INH bulls compared with KLH bulls (P < 0.05). In addition, a significant immunisation × hormone treatment interaction was noted for the number of germ cells. Administration of FSH at the time of initial immunisation against inhibin significantly increased the number of germ cells (92.2 ± 9 × 10⁶ cells) compared with INH+saline bulls (54.9 ± 10 × 10⁶ cells), with INH+GnRH bulls having an intermediate number of cells (64.5 ± 9 × 10⁶ cells; P < 0.05). These results suggest that gonadotropin administration at the time of inhibin immunisation increases the number of germ cells in the testis.