We studied the anti-IgG-induced agglutination of both normal and abnormal red blood cells (RBC) using a sensitive, automated antiglobulin test. Normal RBC agglutinated strongly with anti-IgG antibody, indicating that IgG was present on the erythrocyte membrane. Young RBC, recovered by centrifugation from a normal RBC population, agglutinated with anti-IgG less than the old cells, suggesting that immunoglobulin G accumulated gradually on the RBC membrane in vivo. The degree of anti-IgG-induced RBC agglutination correlated negatively with the reticulocyte count and positively with the concentration of plasma IgG. RBC from patients with hypogammaglobulinemia appeared to have a low subnormal quantity of membrane-bound IgG, whereas the reverse was the case in hypergammaglobulinemia. During hemolytic episodes, RBC of patients with hereditary spherocytosis agglutinated poorly with anti- IgG, apparently due to predominance of young RBC. RBC of patients with nonspherocytic. Coombs-negative, nonimmune hemolytic anemia usually also agglutinated poorly with anti-IgG. However, in some cases of active hemolytic anemia, decreased agglutination with anti-IgG was not observed, suggesting that these young RBC had increased amounts of membrane-bound IgG.