Fc fragments derived from human and murine Ig were found to be potent adjuvants when administered with antigen. Both the in vivo and in vitro anti- sheep erythrocytes (SRBC) responses were significantly enhanced by Fc fragments. The adjuvant effect was shown to be extremely dependent upon the dose of antigen used, with the greatest enhancement occurring when suboptimal doses of antigen are employed. The anti-genicity of the Fc molecule was not related to its adjuvanticity because homologous Fc was as potent an adjuvant as heterologous Fc. Moreover, human Fc fragments enhanced anti-SRBC responses in mice which were tolerant to human gamma globulin.