SUMMARYRabbit erythrocyte antigens A, B, C, F, and H have been demonstrated on rabbit spleen cells, lymph node cells, and three lines of cultured rabbit cells (RK13, LLC-RK1, and R9K5SV40) using the anamnestic response, the mixed agglutination test, and the indirect immunofluorescence technique. These antigens represent factors from four of the six known rabbit blood group loci. When the antigens were used as markers on primary lines of rabbit spleen cells grown in tissue culture, the cultured cells lost the ability to express the antigens. A similar loss was detected in the virus-transformed line R9K5SV40when it was grown in medium containing fetal calf serum in place of the rabbit serum contained in the original culture medium. This loss of antigens seemed to be associated with a decline in the growth of the cultures.