An enzyme-linked immunosorbent assay (ELISA) using monoclonal antibodies has been developed to measure the concentration of three main cellulase components from Trichoderma reesei, cellobiohydrolase I (CBH I), cellobiohydrolase II (CBH II) and endoglucanase I (EG I), in both commercial enzyme preparations as well as in samples from laboratory fermentations. The sensitivity of the assay is 1-10 ng protein, depending on the type of cellulase. The coefficient of variability is between 10% and 20%. By a combination of two different domain-specific monoclonals against CBH I or II it is also possible to quantify the concentration of intact and truncated forms of these two enzymes, respectively. The use of the ELISA to quantify the formation of the three cellulase components under different cultivation conditions is described.