The kinetics and the extent of the conversion of the proenzyme into the active acidic protease (EC 3.4.23.--) of human seminal plasma were dependent on acidic pH. Between pH 2 and 4, the initial rate of the activation was first-order with respect to the proenzyme. Between pH 4.5 and 5, the rate deviated from the first-order with an initial lag period which can be abolished by adding an excess amount of the acidic protease or pepsin. The extent of the activation was complete between pH 2 and 3 and became incomplete between pH 4 and 5. Addition of the acidic protease or pepsin did not alter the extent of the activation at the high pH values. According to the chromatographic profile on a Sephadex G-75 column, the activation products (namely active acidic protease and an activation peptide) obtained at pH 3 and those obtained at pH 4.5 were identical. The molecular weight of the activation peptide obtained at pH 3 was 6900; its amino acid composition was analyzed and compared with those of the proenzyme and the acidic protease. Remarkable similarity between the amino acid composition of the acidic protease and that of human pepsin was observed. In the presence of an excess amount of hemoglobin, the conversion of the proenzyme was self-activated and showed an initial lag period. Addition of acidic protease did not change the rate of self activation or the lag period.