Myelofibrosis is characterized by excessive deposition of interstitial and basement membrane collagens in the bone marrow. In this study, specific radioimmunoassays for the aminoterminal propeptide of type III procollagen and for the 7S collagen domain of type IV (basement membrane) collagen were used to determine how this accumulation is reflected in serum. Of the 41 patients with chronic myeloproliferative disorders studied, the highest levels of both parameters were found in idiopathic myelofibrosis and in chronic myelogenous leukaemia associated with bone marrow fibrosis. Increasing degrees of bone marrow fibrosis were accompanied by increasing serum concentrations of both markers, except for osteomyelosclerosis, where notably low values were seen. Pathologically high values of one or both parameters were also found in a few patients with polycythaemia Vera or a transitional myeloproliferative disorder. The antigens related to type III procollagen and type IV collagen correlated significantly with each other and with the leucocyte count. These parameters should provide noninvasive means for following the accumulation of interstitial and basement membrane collagens in the bone marrow.