Whole antiserum, IgG, and a >99% pure F(ab)2preparation were compared for their ability to enhance Lewis renal allo-grafts in DA recipients. Despite having unimpaired antigen-binding capacity, the DA anti-Lewis F(ab)2was virtually ineffective at the highest dose tested, and was calculated to be a minimum of 100 times less effective than whole antibody. The administration of a 10-fold excess of F(ab)2before an effective dose of IgG did not block the enhancing effect of the latter.