Erythropoietin: hypothesis of action tested by analog computer.

The action of erythropoietin on the pool of undifferentiated bone marrow cells has been examined with the aid of a model tested with an analog computer. The model is consistent with reported experimental results. The essential aspects of the action are (i) the effect is exerted during the S phase of the cell's cycle; (ii) " effective " erythropoietin is present in the cell only during G(1) and part of S; and, (iii) hormone molecules survive in the cell for only a limited time in effective form and require a certain time to assume this form.