It was suggested that the acrosome is a specialized lysosome that has evolved to facilitate, by enzyme action, the sequential steps of penetration of the spermatozoon through the egg investments. The widespread occurrence in acrosomes of acrosin, a proteinase with a pH optimum of about 8 has led to the theory that such an enzyme cannot be accepted as a lysosomal constituent without a fundamental reappraisal of the lysosome concept. The term "modified" has been used to meet the case of a lysosome (i.e. acrosome) containing an unexpected type of proteinase (i.e. acrosin). This blurs the evolutionary aspect and presupposes the existence of a standard lysosome, i.e., one that contains no enzymes other than acid hydrolases.