A decrease of sperm freezability occurred at the K. breeding station, and this situation lasted longer than a year. Out of the 2550 ejaculates taken from 42 breeding bulls within 12 months, 685, i.e. 26.7%, were unfit for use immediately after sperm collection, mostly owing to a low activity of spermatozoa and pathological forms of their motility, and another 469 ejaculates, i. e. 18.3%, were unfit for use after sperm freezing; on the whole, 1154 (i. e. 45.2%) ejaculates had to be excluded. It was revealed by the vital-lethal primuline test that the spermatozoa died quickly after collection. The findings obtained during an electron-microscopic examination of the spermatozoa at the beginning of the process included visible changes in the ultrastructure of the flagellum, particularly its middle piece (deformed shape, incomplete set of axial filaments, vacuolization of the flagellum, abnormal arrangement of the mitochondrial spiral), numerous abnormities of the external cytoplasmic membrane and invagination, vacuolization, and abnormal density of nucleoplasm. The primary changes on the flagella and in the nucleus give evidence that the testicular tissues were altered. The etiological factors behind these processes are believed to include a reduction in the resistance of bulls due to long-lasting consumption of feeds contaminated with the fungus Aspergillus fumigatus, insufficient movement and bad zoo-hygienic practices, all this combined with the secondary action of the infectious germs of Mycoplasma bovigenitalium, which were revealed by cultivation tests in 50% of the ejaculates of the bulls; a positive antibody titre was demonstrated in all bulls.