Most autoantibodies in patients with warm antibody autoimmune hemolytic anemia (AIHA) have specificity within the Rh system. Using rare cells such as -D- and Rhnull cells, Weiner and Vos (1963) described specificity against normal cells (nl), partially deleted cells (pdl), and deleted cells (dl). Recently, autoantibodies which failed to react with Rhnull cells that were of anti-U specificity have been described. It was suggested that the "Rh related" autoantibodies that cannot be identified as specific Rh antibodies may be anti-U. In the present study we examined eluates from the red cells of eight patients with AIHA using a panel of extremely rare cells and cross-absorption and elution techniques. We demonstrated autoantibody specificities not definable without the rare cells and, further, defined heterogeneity of the LW antigen. Autoantibodies with U specificity occurred in three eluates only. It was always present with an antibody of another specificity. Six of the eluates contained anti-LW, two anti-nl, five anti-pdl, three anti-dl, and one anti-e. Absorption and elution studies using the rare Rh-positive LW-negative (Mrs. Bigelow) showed that anti-pdl may in fact represent anti-LW + LW1 and that Mrs. Bigelow may represent a weak variant of LW. Injection of her red cells into guinea pigs produced an anti-LW that reacted similarly to the antibody produced by injecting Rh-positive LW-positive cells. An analogy to the ABO is suggested that normal Rh-positive LW-positive cells represent LW1, Rh-negative LW-positive cells represent LW2, Mrs. Bigelow represents LW3 and Rhnull cells represent the only true LW-negative (lw).