Hemolysin formation was studied in normal rats. It was found that a single intraperitoneal injection of 1 cc. of a 10 per cent suspension of red blood cells results in an optimum high hemolysin titer 5 days after injection. The titer gradually falls, the hemolysin disappearing from 14 to 18 days after the injection. Larger amounts are less effective in the production of amboceptor. Much smaller amounts produce correspondingly lower hemolysin titers, although the ratio is not mathematically proportional. Repeated injections of 1 cc. of a 10 per cent suspension of red blood cells at daily intervals or at intervals of 3 or 5 days for three injections do not increase the hemolysin titer over that resulting from a single injection. Furthermore, the curve of antibody formation following a single small intraperitoneal injec tion of red blood cells is not altered by subsequent injections of similar amounts.