Adult susceptible imice (DBAI2J) iniected with MPSV (myeloproliterative sarcoma virus), a detective RNA tumour virus, develop splenornegaly and progressive disruption of the haematologic system culminating in death. The present study was specifically directed toward determining the effects of t h e 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 ot bone marrow and spleen cells t o form colonies of fully ditferentiated erythroid cells in vitro. MPSV caused substantial modification of both the BFU-E and CFU-E populations in the bone marrow and spleen ot 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 ot the population appeared to be erythropoietin (Ep) independent, although there was no evidence of a