34 patients found to have positive direct antiglobulin tests were divided into 3 classes: (1) no evidence of active acquired hemolytic anemia (AHA); (2) active warm AHA, and (3) active cold AHA. Of 16 patients in the first category, 4 had multiple myeloma,but 10 others showed IgG alone with both χ- and λ-light chains, and 2 others showed both IgG and IgA. None showed C3 or C4. Three patterns of warm AHA were observed:(1) monoclonal IgG without complement; (2) IgA or IgD with C3 but not C4, and (3)multiple immunoglobulins, C3, and C4. In the last group of patients, IgM appeared to be responsible for both PVP-mediated agglutination and fixation at the red cell surface of complement components. The red cells of paroxysmal cold hemoglobinuria and of IgM cold agglutinin disease carried both C4 and C3, but the cells of cold agglutinin disease also carried IgG and IgA. Two cases of mixed cryoglobulin cold agglutinin disease had corresponding immunoglobulins on their red cells plus C3 and C4, and specificity (anti-i and anti-N) was carried by IgG.