In a study of 26 cases of myeloid splenomegaly with myelofibrosis, the authors find a large incidence of immunization anomalies, principally: positive latex reaction in 8 cases; monoclonal proteins in 3 cases; anti-erythrocyte antibodies in 6 cases. Anti-muscle antibodies, quantitative changes of the immunoglobulins and the presence of cryoglobulin are also observed. In a control group of 28 patients of identical age distribution, afflicted with various types of hemopathies, immunization anomalies are encountered far more rarely. The significance of immunization anomalies in the course of myeloid splenomegaly with myelosclerosis is discussed especially in its relationship with the development of medullary fibrosis.