Serum from normal Xenopus, when injected into anaemic Xenopus, causes reduction in both DNA and protein synthesis in erythroid cells as indicated by in vitro culture of the blood cells. Experiments with erythrocyte-conditioned medium, reveal that this inhibitory substance can be recovered from mature erythrocytes. Sephadex G-25 fractionation of normal serum and haemolysate demonstrates that the inhibitory factor consists of molecules of low molecular weight, and experiments with cells of Xenopus kidney reveal that the feedback inhibition may be tissue specific to erythroid cells.

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