Total enzyme activity of whole viscera, and partial characterization of acidic proteases from Monterey sardine viscera are presented. Major proteolytic activity in alkali (pH 10) and minor activity in acid (pH 3) were detected. From purified acidic proteases, six fractions with high activity were selected. One fraction (42) showed one band on SDS–PAGE and two bands on isoelectrofocusing, with pI close to 4.0 and 4.5, respectively. The optimal pH for acidic protease activity was 2.5, with high stability in the acid range and marked loss of activity at neutral and alkaline pH. The optimum temperature was 45 °C, and activity was high at 10 °C, whereas denaturation occurred above 55 °C. Activity was inhibited by Pepstatin A but not by SBTI or EDTA. The general characteristics of these enzymes resemble those of the digestive enzymes of other fish. Because Monterey sardine is abundant in Mexico, it is a potential source for biological reagent production. [Copyright &y& Elsevier]