A case of adult pure red cell aplasia (PRCA) with a serum IgG inhibitor to erythropoiesis and an IgG lambda M component is presented. The study of lymphocyte populations revealed a slight but definite decrease of E and EA rosettes, with dissociation between E rosettes and PHA blastic transformation of blood lymphocytes and increase of membrane IgM-bearing lymphocytes. The relationship between PRCA and paraproteinemia is discussed: it is suggested that the serum M component may derive from an immunological unbalance between T and B lymphocytes. Since a survey of the literature reveals 5 similar cases, it is suggested that paraproteinemia may be the hallmark of a particular variety of chronic PRCA 'type 1'.