Serum collected at intervals from rabbits treated with Freund's complete adjuvant was injected together with fluid diphtheria toxoid into recipient rabbits. Control rabbits received toxoid alone or toxoid mixed with serum of untreated donors. There was no antibody response in the controls or in recipients of serum obtained from donors 3 days after adjuvant treatment. Recipients of serum obtained from donors 1 to 9 weeks after adjuvant treatment responded with antibody formation. The magnitude of the response of recipients was correlated with the increase in gamma-globulin concentration in the serums of the corresponding donor rabbits. Passive transfer of adjuvant action indicates that a serum factor, possibly natural antibody, is partially responsible for the immunity-enhancing activity of Freund's adjuvant.