Chromatin decondensation of human ejaculated spermatozoa was studied in vitro, at various points of time after ejaculation, by sperm exposure to the detergent sodium dodecyl sulphate (SDS) containing zinc chelating EDTA. Within 5 min after ejaculation EDTA revealed a capacity for decondensation in 90% of the spermatozoa. This sperm capacity decreased rapidly upon storage. The results support the concept that the capacity to decondense is a normal property of freshly ejaculated spermatozoa and that this property may be rapidly lost. The loss is most probably due to an inability of thiol groups to take part in a thiol-disulphide exchange in the sperm chromatin. A loss of functional thiols may hinder a capacity for chromatin decondensation inherent to the spermatozoon. A loss of thiols due to oxidation, that is, surplus S-S bridge formation, may also delay hypothetical extrinsic S-S cleaving factors in the ooplasm. In either case, the complete and non-delayed sperm chromatin decondensation in the ovum may be hindered. This may result in the abnormal embryonic development observed in ova fertilized with aged spermatozoa.