SUMMARYIn mice treated with antilymphocyte serum (ALS), the curve relating dose of serum to number of antibody-forming cells or graft-rejecting cells is hyperbolic, i.e., log cell number is inversely related to log dose. As a consequence, it is shown on theoretical grounds that, if a fixed total amount of serum is administered over a fixed total period, regimens of small frequent doses should be, within limits, more effective than regimens of large, infrequent doses. This prediction has been confirmed by comparing the effects of different regimens of ALS on the formation of hemolytic plaque-forming cells (PFC) in the mouse spleen after injection of sheep red cells and on rejection of skin grafts.