pH-induced changes in mitotic and developmental patterns in sea urchin embryogenesis. II. Exposure of sperm.  pH decrease and increase were tested for their ability to affect the fertilizing capacity of sea urchin sperm, as well as to induce developmental defects and mitotic abnormalities in the embryos generated by pretreated sperm. Seawater (sw) at different pH values was obtained by mixing acidified (HCl) sw and alkalinized (KOH) sw. Thereafter sperm were exposed to different pHs for a defined time interval, or subjected to inactivation, while suspended in sw at defined pHs, as long as their fertilizing capacity was maintained. The study was carried out over the pH range 5-9; sperm showed optimal fertilizing capacity between pH 6 and 7. Below pH 6 and above pH 8 a drop in the fertilizing capacity was observed, whereas at normal sw pH (8.0-8.2) sperm showed intermediate values. pH decrease in sperm suspension induced a reproducible increase in developmental defects and mitotic abnormalities in the offspring of pretreated sperm. This effect displayed a dose-response relationship, which was most evident for pH ranging from 7 to 8. The effects of decreased pH were detected as well in terms of quantitative changes in mitotic activity, which decreased in embryos generated by sperm exposed to low pH (5-6). The results are consistent with the induction of genetic damage following exposure of sperm to low pHs. This observation suggests a genotoxic action of hydronium ions per se. Another possible explanation for the observed genotoxicity of decreased pH might be based on the pH-dependent activity of several genotoxins present in sw at trace levels.