The effects of ultrasmall (2-3 nm) gold nanoparticles on native epididymal sperm chromatin of CBAxC57BL/6 hybrid mice and 129/IMG mice with a mutation in the DNA-polymerase iota gene were studied. It is shown that for both mouse strains after sperm incubation in a solution containing Au nanoparticles, at 23, 37 and 60 degrees C for 30 min followed by 1 hour treatment in dithiothreitol solution, a decrease in the number of nuclei with fully decondensed chromatin was observed compared with the control. Though, the manifestation of this effect in the population of 129/IMG mice mature sperm, was weaker. Also we have demonstrated that sperm of both strains that were incubated in a sol of Au nanoparticles at 60 degrees C behave differently under the action of dithiothreitol. A considerable part (-80%) of sperm of CBAxC57BL/6 hybrid mice treated with Au nanoparticles showed high resistance to the action of dithiothreitol, whereas in the case of 129/IMG mice only -30% did, and a partial or complete chromatin decondensation takes place in the remaining sperm. In general, using the method of nuclear chromatin decondensation in vitro for the native sperm, the patterns that we have identified in earlier studies on previously demembranized sperm are confirmed.