We have characterized the proteolytic activities (proteases in cooperation with carboxypeptidases) involved in the different stages of germination of maize (Zea mays L.) grains. A sequential expression of different groups of proteases (aspartic, cysteine, serine and metallo-proteases) with pH optima in the acidic range has been found by using specific protease inhibitors. Pepstatin-sensitive proteolytic activity (aspartic-protease activity) is dominant in resting grains. Germination is accompanied by the appearance of a proteolytic activity which can be enhanced by low-molecular-weight thiol compounds and inhibited by thiol-protease inhibitors, which is indicative of the involvement of cysteine protease(s). This burst of cysteine-protease activity is coincident with the disappearance of the main storage-protein fractions. We conclude from this that cysteine protease(s), with an acid pH optimum, are good candidate(s) for the proteolytic attack of stored protein reserves in maize. After this stage, where cysteine-protease activity is dominant, a period with larger total proteolytic activity starts, coincidentally with the expression of the different types of other proteolytic activities (serine, aspartic and metallo proteases), in addition to the cysteine-protease activity above mentioned. When the development of carboxypeptidase activity during germination was analyzed, the highest activities were found during the earlier and later stages. This result is indicative of a cooperative interaction between carboxypeptidase and endoproteolytic systems in order to obtain a more effective mobilization of storage proteins in germinating maize grains. The phytohormones, gibberellic acid (GA3) and abscisic acid (ABA) which can stimulate or inhibit, respectively, the total proteolytic activity in extracts from germinating grains, exert a differential effect on the different proteolytic activities here detected.