A new attempt was undertaken to check the reproductive capabilities of male hybrids bred from European bison (Bison bonasus) and domestic cattle (Bos taurus). The analysis concerned two hybrids aged 1.5?2.5 years (the latter being sampled twice at 6-month intervals), and additionally one 1.5-year-old backcross hybrid. Examination of testicular tissue revealed halting of spermatogenesis, which was hardly ever able to proceed further than meiosis I. Spermatogenesis was arrested at the stage of spermatogonia and primary spermatocytes. Spermatozoa were found only in the case of one hybrid and these were mostly malformed. To summarize, all the males studied were sterile, despite the evident sexual behavior exhibited by one individual.