This study is an attempt to define the antigenic composition of rat liver cell types in order to establish a basis for describing liver specificity in immunological terms. Rat liver parenchymal (PAR) and reticuloendothelial (RE) cells were separated by centrifugation through a sucrose medium of elevated density. Homogenates of the separated cell types were used to raise antisera in rabbits. The antisera were tested against various antigen preparations in agar diffusion experiments. It was noted that there is a large number of antigens in PAR cells that are not detectable in RE cells. Conversely, RE cells contain at least two antigens absent in PAR cells. These antigens were not detected in rat serum, red blood cells or lymphocytes. Absorption experiments showed that while certain PAR antigens were common with those of other organs, at least four PAR antigens were not shared with the other organs tested here (kidney, testis, lung, heart and spleen). The antigens were further characterized by staining the immune precipitates for nonspecific esterase activity. PAR extracts contained at least five distinct antigens with enzyme activity. Of these, two were also present in rat kidney and two others in rat testis. At least one of these antigens was found only in liver. An acid phosphatase active antigen was found in PAR cells, but not in RE cells. This antigen was not specific for liver. Fluorescent antibody staining of tissue sections substantiated the specificity of the PAR and RE antisera.