In this study the distribution of type IV collagen in the marrow is compared with that of laminin, another basement membrane protein. In addition, incompletely processed type III procollagen is identified with specific antibodies. In normal bone marrow the distribution of the type III procollagen antigen closely resembles that of reticulin staining. In all the myelofibrotic samples, representing both early and advanced disease, the fibrous tissue stains heavily for this antigen. Thus type III procollagen which has not completely lost its aminoterminal propeptide is a genuine component of the extracellular matrix fibres in human bone marrow. Laminin is found with type IV collagen in continuous basement membranes in arterial walls, whereas only discontinuous strips of staining are seen along the sinusoids in normal marrow. In myelofibrosis the dilated or obliterated sinusoids have thickened or continuous basement membranes, visible with both stainings. Neovascularization also increases the extent of basement membrane staining in fibrotic marrow. With respect of these antigens, there is no difference between primary and secondary myelofibrosis. These changes warrant the use of serum antigens related to type IV collagen and to type III procollagen as markers for developing myelofibrosis.