Chromatin stability is an important determinant of semen quality, essential for spermatozoa maturation in epididymes and early embryogenesis. A radioisotope method based on the quantitative measurements of tritium-labelled actinomycin D (3H-AMD) incorporation into the spermatozoa nuclei was used to assess chromatin stabilization of boar spermatozoa incubated with physiological (reduced glutathione – GSH, heparin – H and bosine serum albumin – BSA) or non-physiological (dithiothreitol – DTT, disodium ethylenediaminetetra acetate – EDTA, 2-mercaptoethanol – ME and sodium dodecyl sulphate – SDS) decondensing agents.The effect of the composition of seminal plasma and the role of zinc ions in chromatin stability of spermatozoa were also studied. Pre-treatment of spermatozoa with GSH, H, DTT, ME or SDS resulted in an increase in the incorporation of 3H-AMD into the spermatozoa nuclei. In contrast, when sperm samples were treated with BSA or EDTA there was a reduction in the incorporation of 3H-AMD, what was attributed to hyperstabilization of chromatin. A presumed hyperstabilization was also observed when SDS+EDTA+H were used. On the other hand, an exceptionally strong action of decondensation of chromatin was induced by H+BSA. Increased incorporation of 3H-AMD into the spermatozoa nuclei was concomitant with low zinc and protein content in the seminal plasma of boars following depletion test (DT), suggesting disturbances in chromatin stability. The presented radioisotope method based on the application of 3H-AMD is a simple and reliable assay that can be used to monitor the chromatin status of boar spermatozoa.