Smear-ripened cheese varieties are characterised by the growth of a smear culture, containing predominantly Brevibacterium linens, on the cheese surface during ripening. In such cheese, considerable zonal differences in biochemistry of ripening exist, due to moisture loss from, and growth and metabolic activity of smear microflora at, the cheese surface. In this study, the effects of adding exogenous plasmin or small amounts of mastitic milk to good quality milk on the quality of smear-ripened cheese made subsequently was examined. Addition of plasmin did not influence cheese composition immediately after manufacture, but slightly decreased the rate of moisture loss during cheese ripening. Plasmin activity decreased during the early stages of ripening, but subsequently increased towards the end of ripening, perhaps due to changing pH conditions in the cheese. Addition of plasmin increased rates of primary proteolysis in cheese, as measured by levels of pH 4.6-soluble N and urea-PAGE, although production of later products of proteolysis appeared less affected. Addition of mastitic milk had largely similar effects to addition of exogenous plasmin, which may reflect a high content of plasmin or plasminogen activators in such milk. Overall, changes in milk quality and enzymology appear to influence the quality of smear-ripened cheese.