Human sperm nuclei show an exceptional variability; compared with sperm nuclei of other Eutherian mammalian species. The variable stability is caused by a variable content of chromatin stabilizing disulphide bridges, which in turn is determined by differences in composition of basic proteins: protamine types and subtypes, histones, and intermediate forms. An abnormal state of the chromatin can be related to an abnormal DNA configuration or an abnormal DNA content: diploid instead of haploid. Abnormality of the nuclear chromatin is probably one of the causes of morphological aberrations of the sperm head. A relationship between abnormal chromatin and male infertility has been reported repeatedly. Recently available evidence suggests that living spermatozoa with abnormal chromatin, leading to abnormal morphology, have a strongly reduced capacity to fertilize an oocyte. In addition, if these spermatozoa fertilize oocytes, the embryonal development will most probably be abnormal.