Heterospecific in vitro fertilization necessitates the completion of the capacitation, the acrosome reaction, and the nuclear decondensation into the ooplasm. Using 10 semen samples the results of this test using zona-free hamster eggs were compared with the results of a nuclear chromatin decondensation (NCD) ability test in vitro using 1% SDS + 6mM EDTA. The results showed that there is a positive correlation between fertilization rate and NCD ability of migrated/capacitated spermatozoa. It is postulated that an hyperstabilized spermatozoon (with a high rate of disulphid bridges between chromatin fibers) cannot decondense either in an hamster oocyte or after SDS/EDTA, whereas a normal chromatin stabilized spermatozoon (with a good rate of SH-Zn-SH links and S-S links) has a normal NCD rate and thus a good fertilization ability.