The amino-terminal fragment of procollagen III was assayed by RIA in 186 patients with myeloproliferative disorders, at intervals of 6 to 24 months. The present results suggest that this assay may be useful for discriminating primary and secondary erythrocytosis, for evaluating the evolution of polycythemia vera towards spent phase and/or myelofibrosis, and for estimating the degree of myelofibrosis in patients with myeloid metaplasia of the spleen. However the methodology of the assay still needs technical improvements. An assay of procollagen I is needed. Excessive values of serum procollagen III may be due to hepatic or pulmonary fibrosis. Finally we don't have as yet a follow-up delay long enough to ascertain the prognostic value of the assay in polycythemia vera and in primary myelofibrosis.