SummaryPlasmin (EC 3.4.21.7) was added to cheese milk to assess its contribution to Cheddar cheese ripening; the activity of plasmin in the cheese was increased by levels ranging from 1·5 to 6 times that in the control cheeses. Even at the highest level of added plasmin, no activity was found in the whey. β-Casein was degraded faster in the experimental cheeses than in the controls, and the concentration of γ-caseins increased concomitantly. The total N in the water-soluble extract was up to ˜ 20% higher in the experimental than in the control cheeses but phosphotungstic acid-soluble N was not affected by the plasmin activity in the cheese. Several differences were apparent in the gel electropherograms of the water-soluble extracts of the experimental and control cheeses; some peptides were present at higher concentrations in the former, others in the latter, suggesting that plasmin contributes to both the formation and degradation of water-soluble peptides in cheese. The organoleptic quality of the plasmin-enriched cheeses was judged superior to that of the controls and ripening was considerably accelerated; a plasmin level 3–4 times the indigenous value appeared to be optimal. No bitterness was detected in any of the cheeses.