The immune response to bacteria and to a soluble protein was compared in germfree and conventionalized mice. Sixty germfree and 59 conventionalized mice received a suspension of killed Serratia marcescens into one front foot-pad and sterile horse ferritin into the other and were sacrificed in groups from 2 hr to 14 days after inoculation. All mice had no pre-existing antibody to either antigen and the flora of the conventionalized mice never contained Serratia. Lymphatic tissue changes and the fate of the antigens were followed in axillary lymph nodes and the spleens by histologic, fluorescent antibody, and autoradiographic techniques after tritiated thymidine injection. Individual serum antibody titers for both antigens were determined at each time period. The cellular and serologic responses were slightly delayed in the germfree mice but later equaled and sometimes exceeded those of the conventional animals. In all animals, lymph nodes draining the site of Serratia injection showed a more vigorous response than those on the ferritin-injected side but the reaction was qualitatively the same for both antigens. All lymph nodes contained the antigens by 2 hr after foot-pad injection. With time, both antigens lost their particulate nature sooner in conventionalized than in germfree macrophages. In the latter, both antigens persisted throughout the study while no longer demonstrable with fluoresceinated antiserum in conventional macrophages after the first week. While phagocytosis is equal in germfree and conventional mice, a greater digestive capacity of macrophages for antigens seems to result from the continuous exposure of conventional animals to the immunologic effects of the microbial flora. Conversely, the lack of substantial antigenic stimulation of lymphatic tissue in germfree animals fails to develop these macrophage functions beyond their basic ability to degrade foreign substances. Although the onset of the immune response is delayed in germfree mice, the relatively prolonged antigen digestion and the presumably slower release of immunogenic antigen fragments result in a more sustained and sometimes greater response than in conventional animals. This modifying effect of the microflora on the function of macrophages during the immune response is independent of previous experience with, or the nature of, the antigen.