A fibrin clot culture system with high plating efficiency is described for the growth of erythroid cells from chick bone marrow. Erythroid colonies grown in the absence of adult chicken plasma (spontaneous colonies) were either benzidine-negative or weakly benzidine-positive. Colonies grown in the presence of chicken plasma were 90% strongly benzidine-positive and 40% more abundant than spontaneous colonies. Plasma from anemic chickens was more effective than control plasma in inducing heme accumulation (heme-stimulating activity) and in increasing the number of erythroid colonies (colony-stimulating activity). Spontaneous colonies from 48-h cultures were transformed into benzidine-positive colonies by exposing them for 6-10 h to chicken plasma.

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