The cysteine proteinase from Micrococcus sp. INIA 528 was added at three concentrations to pasteurized cows' milk to study its effect on the ripening process of Hispanico cheese. Dry matter was higher in whey from experimental cheeses than in whey from control cheese manufactured from milk with no added proteinase. No differences in cheese proteolysis were found 24 h after manufacture. Residual αs- and β-caseins were significantly lower, and N soluble at pH 4·6 and in trichloroacetic acid significantly higher, in experimental cheeses than in control cheese from day 15. Hydrophobic and hydrophilic peptides in the water-soluble fraction of cheese measured by absorption at 280 nm were significantly higher in experimental cheeses than in control cheese from days 1 and 15 respectively. Texture was less firm in experimental than control cheese from day 30. Bitterness, detected in cheese with the highest levels of added cysteine proteinase, adversely affected cheese flavour. Flavour intensity was significantly higher from day 30 in cheeses with added proteinase than in control cheese.