Hemolysate from adult and newborn rats was fractionated into nine components (I-IX) using DEAE-cellulose chromatogra-phy. A subsequent chromatographic step resolved component IV into two, IVa and IVb. A comparison of adult and newborn rat hemolysates showed that newborn had higher levels of components I, 11, and IV. In the newborn, they accounted for 52% of total hemoglobin eluted, compared to 34% in the adult. Components V, VI, and VIII were less in the newborn than in the adult red cell, at 42% compared to 59%. Minor components 111, VII, and IX were unchanged, at 5.8% in newborn and 5.3% in adult rats. Bone marrow cells from adult rats cultured for 46 hours, showed stimulation of eight components under the influence of erythropoietin (components IVa and IVb were not distinguished, and VII was not detected). The extent of stimulation was not consistently higher for major than for minor hemoglobins. These data show that adult rats possess at least ten hemoglobin components, that newborn rats have the same components but in different proportions, and that both major and minor components are stimulated by erythropoietin in bone marrow cell cultures.