The influence of 48-h treatment with epoxomicin, an inhibitor of proteolytic activity of proteasomes, at the concentration 10 microM, on spermiogenesis in algae Chara vulgaris was examined. In the presence of the inhibitor, the frequency of early spermiogenesis phases significantly increased, the number of spermatids in mid-phases decreased and disappearance of late phases was observed. A hypothesis has been put forward that epoxomicin stops spermiogenesis during the period of preparation to further deep reorganisation of spermatids by blocking proteolysis of short-lived regulatory proteins which are responsible among others for triggering the exchange of nucleohistones into nucleoprotamines.