The responsiveness to ESF of the erythremic mouse is increased one to four days after a previous stimulation with ESF. The formation of nucleated red cells by the priming ESF stimulation was suppressed either by simultaneous injections of ESF and Actinomycin D, or by giving the priming ESF challenge to erythremic mice shortly after they had been exposed to 400 R. total body irradiation with X-rays. By these techniques it was demonstrated that the presence of nucleated red cells within the erythron is not important for the observed increase in dose response of ESF after ESF stimulation. It was concluded that the recruitment of primitive erythropoietin-responding cells is enhanced by ESF.

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