A sandwich-type ELISA has been developed for quantification of the complex between the cysteine proteinase cathepsin Β (CB) and its reversible tight-binding inhibitor cystatin C (CC) in normal and pathological sera. The assay is based on a combination of catching Ab (3E1), raised against CB, and a horseradish peroxidase-labelled detection Ab (1A2), raised against CC. Only the CB/CC complex is able to evoke a signal in this assay. The detection limit of the assay was 15.5 ΠΜ and the working range between 31.3-200 nM. The within and between-run coefficients of variance (CV) varied from 4.7% to 9.4% and 11% to 12.8%, respectively, demonstrating satisfactory reproducibility of the method. The concentration of the CB/CC complex was determined in sera from 90 healthy controls, 32 patients with non-cancerous lung diseases, 148 patients with lung and 32 patients with colorectal cancer. The CB/CC complex was significantly less abundant in sera of patients bearing malignant lung tumours than in those with non-cancerous lung diseases or healthy controls (p<0.001). In colorectal cancer sera its level was significantly lower in advanced stages C and D than in early Dukes' stages A and Β (p=0.02). Our results show that the increased levels of CB in malignant sera are not impaired effectively by CC and support the hypothesis of hindered inhibitory capability during cancer progression.Keywords: Cathepsin B/Colorectal Cancer/Complex/ Cystatin C/Lung Cancer. Yan, S., Sameni, M. and Sloane, B.F. (1998). Cathepsin Β in human tumor progresión: a review. Biol. Chem. 379, 113-123. Warwas, M., Haczynska, H., Gerber, J. and Nowak, M. (1997). Cathepsin B-like activity as a serum tumor marker in ovarian carcinoma. Eur. J. Clin. Chem. Clin. Biochem. 35, 301 -304.