We have investigated the activities of alloantisera produced in B6 (C57BL/6) and B6.S strain mice reciprocally immunized with unwashed bone marrow and spleen cell suspensions from their respective Fv-2 congenic partner strains, B6.S and B6. These antisera inhibited the formation of colonies by the late erythrocytic progenitors (CFU-E) in plasma cultures seeded with unwashed bone marrow or spleen cells; washed cells were unaffected. Erythropoietic burst formation by the early progenitors (BFU-E) was not significantly inhibited by the antisera, whether the cells were washed or unwashed. We conclude (a) that the congenic antisera are capable of recognizing alloantigens controlled by alleles of Fv-2 or of a closely linked gene locus on chromosome 9; (b) that these alloantigens are situated on the surface of erythrocytic progenitor cells and can be removed by washing; and (c) that the expression of the alloantigens on these cells is influenced by their stage of differentiation.

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