Cryptorchid bulls have low economic value owing to the effects of masculinization. Moreover, surgical removal of an ectopic testis is difficult in certain clinical cases. Recently, immunocastration has garnered popularity as a nonsurgical castration method in pig farming; however, the effects of immunocastration on cryptorchid bulls are yet to be yet. Herein, we investigated endocrine changes due to immunocastration in cryptorchid bulls and studied its effectiveness. This study included 13 Holstein bulls diagnosed with cryptorchidism and classified into two groups based on pubertal period: <8 months of age (pregroup) and ≥8 months of age (postgroup). Antigonadotropin-releasing hormone (GnRH) vaccine was used for immunocastration, and two vaccine doses were administered. Blood testosterone and anti-Müllerian hormone (AMH) levels were measured and analyzed for endocrine evaluation. The testosterone levels significantly decreased following the start of immunocastration in both groups, thereby confirming the efficacy of antiGnRH vaccination in cryptorchid bulls. The AMH levels significantly increased in the pregroup with two antiGnRH vaccination, suggesting a compensatory response via the neutralization of GnRH antibodies. The AMH levels did not significantly change in the postgroup, indicating the partial suppression of AMH secretion in Sertoli cells during sexual maturation and failure of Sertoli cell maturation. Thus, we successfully restrained the serum testosterone levels in cryptorchid bulls using antiGnRH vaccine. The testosterone levels are a useful indicator of the immunocastration effect on cryptorchid bulls.Hereafter, a vaccine program that can sustain the castration effect on cryptorchid bulls is necessary.