Erythrocytes from the spleen of CBA mice have been prepared for analyses by flow cytometry. About 80% of the polychromatic erythrocytes (PCE) in the spleen originate from erythropoiesis in the spleen, while the remaining 20% come from the peripheral blood. Analyses of the RNA content of PCE revealed that splenic PCE do not mature into normochromatic erythrocytes (NCE) in the spleen but leave the organ at a more immature stage. A considerable part of the PCE from bone marrow also mature into NCE in the bone marrow. The rate of RNA breakdown in PCE follows an exponential function. Time-courses for the appearance of micronucleated PCE (MPCE) from spleen and from bone marrow were determined by analysis of samples taken with short intervals after an acute dose of 0.1 Gy X-rays. The time-courses were identical for MPCE from the spleen and the bone marrow: The frequency of MPCE (fMPCE) starts to increase at about 10 h after irradiation and reaches its maximum after about another 20 h, upon which fMPCE returns to control level. The first induced MPCE in peripheral blood appear at about 20 h after irradiation. The effects of the carcinogen DMBA, 9,10-dimethyl-1,2-benzanthracene, at low doses were determined in PCE from spleen and bone marrow. The sensitivity was found to be about the same for erythroblasts in the spleen and the bone marrow. Protracted exposure to γ-irradiation at a very low dose rate (44 mGy/day) gave a similar increase of fMPCE in bone marrow and spleen. The suitability of using splenic erythrocytes in the micronucleus test is discussed.