A newly developed collagenase assay method using (14)C-labeled soluble collagen as substrate has been applied to the determination of collagenase activity in human leucocytes and synovial fluids. The results indicated that collagenase activity in the both samples could be measured by this method as observed with tadpole collagenase, except that 35 % dioxane was preferable for the extraction of the enzyme digestion products in the case of synovial fluids. Collagenase activity determined by the present method was well correlated with that obtained by the conventional assay method using reconstituted collagen fiblils as substrate (r=0.963 and 0.829 (p<0.01) respectively). In preliminary experiments with leucocytes from patients with diabetes mellitus (DM), systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), it was observed that collagenase activity in leucocytes increased significantly in SLE and from some patients with RA, while no appreciable differences were observed in diabetics, compared to that in normals. In synovial fluids from patients with RA and osteoarthritis (OA), the accumulation of large amount of inactive (latent) form of collagenase was demonstrated by the treatment of synovial fluids with 3M KI. And it was varied significantly in one patient at different time while no appreciable amount of the active form was observed during the period.