The distribution of collagen type VI and tenascin has been determined in both normal and myelofibrotic bone marrow by immunohistological techniques. In normal sections positivity was demonstrated in the periosteum (collagen type VI and tenascin) and in the walls of small blood vessels (tenascin). In contrast, myelofibrotic bone marrow showed an increased deposition of both proteins, especially collagen type VI, although this increase was restricted to the later fibrotic stages of the disease. Serum concentrations of collagen type I (PICP), collagen type III (PIIIP) and laminin (laminin PI) related polypeptides were determined in a further 26 patients. PIIIP levels were significantly raised, in contrast to PICP and laminin PI concentrations. All three markers, however, were significantly elevated in patients with active/transforming disease. Laminin PI and PICP levels showed a strong correlation, indicating a relationship between basement membrane and interstitial collagen metabolism, although they do not appear to offer any advantage over PIIIP for the monitoring of disease activity.