Enzyme linked and radioimmune direct antiglobulin tests (DAGTs) were used to assess red cell bound IgG, IgA, and IgM in 585 patients referred to an immunohaematology reference centre. One hundred and fifty eight patients with less than or equal to 200 mol IgG and small amounts of IgA and IgM coating their red cells were studied in detail. The presence of autoimmune haemolysis was determined from the clinical, haematological, and biochemical findings; it occurred in at least 25% of the 158 patients, the degree varying widely. There was a highly significant association between small increases in cell bound immunoglobulins and the presence of autoimmune haemolysis. Immunoglobulins of IgG, IgA, and IgM classes could produce autoimmune haemolysis when the classical agglutination DAGTs were negative; the IgA and IgM were usually found in association with IgG. The haemolytic effect was enhanced by the presence of complement and combinations of immunoglobulin classes on the red cells.