The stability and the content of zinc of the chromatin were studied in spermatozoa from ten men with unexplained infertility, and in spermatozoa from five fertile donors. A positive relation was found between zinc in sperm nuclei (X-ray microanalysis) and the resistance of the chromatin to decondense in sodium dodecylsulfate (SDS). The infertile men had lower degree of sperm chromatin stability and lower sperm zinc content than the fertile donors. A subgroup of the infertile men, which all had minor clinical signs of prostatic inflammatory reaction, had the lowest content of zinc in the chromatin and the lowest degree of chromatin stability. A low content of nuclear zinc would impair the structural stability of the chromatin and thereby increase the vulnerability of the male genome. This mechanism may be one explanation for the reduced fertility of the men with minor inflammation of the prostate.