Peroxiredoxins (Prx) are a recently identified family of proteins that have been shown to exhibit peroxidase activity, as well as other divergent functions. Of the six known members of the family, Prx4 is present as a secretable form in most tissues and as a membrane-bound form only in testes. We recently proposed that the unprocessed form may be involved in acrosome formation during spermiogenesis. In the present study, it was found that levels of the unprocessed Prx4 decreased during cryptorchidism and this decrease corresponded to the degree of the defect in spermiogenesis. In contrast, the levels of the secretable Prx4 remained virtually the same during cryptorchidism. These results were consistent with selective expression of the unprocessed form after the spermatid stage, suggesting the existence of a relationship with acrosome formation.