Chronic myelogenous leukaemia (CML) is frequently accompanied by progressive myelofibrosis that is probably largely due to increased collagen production by bone marrow fibroblasts or decreased collagen degradation by collagenase. We investigated the activity of collagenase and collagenase-inhibitory factors (CIF) in serum and bone marrow fibroblast-conditioned medium obtained from patients with CML and healthy volunteers. The activity of both collagenase and CIF was significantly higher in the CML patients than in the normal volunteers. Our previous study showed that bone marrow fibroblasts from CML patients produce significantly more collagen than fibroblasts from normal subjects. Therefore, it appears that the in vitro production of collagenase and CIF by bone marrow fibroblasts is related to the degree of in vivo collagen production. and that both the increased collagenase and CIF activities are reflections of collagen overproduction in CML.