The capacity of blood and lymphatic cells and of the spleen in rabbits immunized with sheep red blood cells to produce antibodies (PFC) has been studied. In rabbits in which only the blood was examined, the numbers of PFC reached a peak 4 days after one immunization, or 3 days after two immunizations. Moreover, double immunization yielded 10 times higher numbers of PFC than single immunization. Comparative studies of the blood, lymph and spleen revealed largest numbers of PFC in the spleen, smaller in the blood, and fewest in the lymph. This pattern was observed after single as well as double immunization, although the proportions were different after double immunization. After the second immunization much larger numbers of PFC appeared, increasing especially in the lymph. Discrepancies between the findings and earlier results, and the problem of the mechanisms of permeation of immunologically competent cells into the blood are discussed.