Bovine plasmin (PLM) and its precursor plasminogen (PLG) are present in cheese, in which the proteinase is thought to play a significant role in proteolysis during ripening. PLM in cheese is usually assayed by enzymic techniques and concentrations are generally expressed as activities. In order to determine absolute concentrations of PLM and PLG in cheese, we have applied an ELISA previously developed for the differential determination of PLM and PLG in milk. The assay used two monoclonal antibodies, one specific for PLG and the other cross reacting with PLM and PLG. PLM concentration was obtained by subtracting the PLG concentration from the value for PLM plus PLG. The assay was evaluated in cheese by determining the total amount of PLG and PLM in milk used in the manufacture of eight semi-hard cheeses on a pilot plant scale, and comparing it with the total amount of PLG and PLM present in the eight corresponding wheys and unripened cheeses. Concentrations of PLG and PLM in 19 commercial cheeses were also determined. It was thus demonstrated that the ELISA made it possible to quantify PLM and PLG in milk and cheese and that PLM and PLG concentrations in cheese were highly dependent on the technology used for cheesemaking.