Mice were given injections of rat Tamm-Horsfall protein (TH) in order to study immune complex formation in the distal nephron. After immunization, all mice had high serum levels of antibodies to TH; immune deposits consisting of IgG antibodies to TH and TH formed at the base of cells of the thick ascending limb of the loop of Henle (ALH) and distal convoluted tubule. These basal complexes were maximal in number and size within the cortical ALH, where they often radiated toward the luminal surfaces. Although the highest anti-TH antibody titers were found in animals with the most extensive deposits, antibody levels were not directly proportional to IgG deposits or to the time after immunization. The larger immune deposits were visualized by light microscopy as PAS-positive deposits. The quantity of these deposits was proportional to the time after immunization and was directly related to anti-TH antibody levels. Electron microscopy showed that these immune deposits were present within the basal and lateral intercellular spaces of the cells of the ALH. The distribution and localization of TH within the normal mouse kidney was very similar to that in rats. However, the distribution of immune complexes within the distal nephron in mice differed from rats similarly treated with TH and indicates either species differences in the distribution and/or organization of TH associated with tubular cell surfaces or that the accessibility of distal tubules to antibodies is species-dependent.