AbstractAdult susceptible mice (DBA/2J) infected with MPSV (myeloproliferative sarcoma virus), a defective RNA tumour virus, develop splenomegaly and progressive disruption of the haematologic system culminating in death. The present study was specifically directed toward determining the effects of the virus on erythroid differentiation. Early and late precursor cells (erythroid burst‐forming units; BFU‐E and colony‐forming units; CFU‐E, respectively) were evaluated by the ability of bone marrow and spleen cells to form colonies of fully differentiated erythroid cells in vitro. MPSV caused substantial modification of both the BFU‐E and CFU‐E populations in the bone marrow and spleen of infected animals. Changes were detected in the CFU‐E population preceding any significant increase in spleen weight. In the bone marrow, the proportion of CFU‐E cells increased almost twofold by days 5–10 after virus infection but decreased by day 15. In the spleen, CFU‐E frequency rose 40‐fold by days 10–15 and then declined steadily prior to death. At the peak of CFU‐E expansion, a small proportion of the population appeared to be erythropoietin (Ep) independent, although there was no evidence of a complete switch to Ep‐independence which occurs in Friend virus‐induced erythroleukemia. Dose‐response curves showed that none of these data could be explained in terms of a changing responsiveness to Ep. However, evidence is presented that indicates that BFU‐E from MPSV‐infected animals lose or have a reduced requirement for burst‐promoting activity (BPA) relative to normal cells although their progeny still need Ep f
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