Macrophage-bound antigen inoculated into footpads of rats induced a state of delayed hypersensitivity (DH) equivalent to that induced by free antigen but less than that induced by antigen incorporated into complete Freund's adjuvant. Antigen bound to macrophages was more immunogenic than free antigen and elicited antibody responses in all injected rats. Macrophages injected intradermally into rats immunized with antigen in complete Freund's adjuvant excited an acute inflammatory reaction consisting of numerous polymorphonuclear leucocytes and abscesses. In the same animals free antigen produced typical DH lesions, and antigen bound to macrophages produced a mixture of polymorphonuclear and mononuclear cell infiltrates. Intradermal deposit of macrophages or macrophages with antigen increased the number of basophils in areas of inflammation. Basophils did not vary in number or distribution in DH lesions elicited by free antigen at 14 or 28 days after inoculation.