Spermatozoan nuclear chromatin is in a highly condensed state prior to fertilization. In vivo decondensation occurs in the ooplasm and is essential for successful fertilization and the formation of male pronucleus and the zygote to occur. The chromatin of spermatozoa and nucleus can undergo in vitro decondensation with sodium dodecyl sulfate (SDS) and 6 mM ethylene diamine tetraacetic acid (EDTA). The ability of sperm to decondense in vitro was compared with their ability to fertilize human oocytes in vitro. Spermatozoa from normal samples were studied for their decondensation ability as regards their fertilizing performance in an in vitro fertilization (IVF) program. Fertilization occurred when the decondensation percentage of sperm nuclear chromatin was more than 70%. The effective sperm count was significantly (p less than 0.05) lower in the unfertilized group. This is a new diagnostic technique to assess sperm-fertilizing potential at the initial evaluation of the male.