A transplantable reticulum-cell sarcoma induced by Rauscher virus (RV) in (C57BL/6 X DBA/2)F1 (BDF1) mice was grown in tissue culture. Four separate cell lines were established, all of which grew predominantly in suspension. The doubling time of the cells from these cultures ranged from 17 to 32 h. Each culture continued to replicate RV, as indicated by the infectivity in newborn mice of all fluids tested up to the 75th passage. Since the morphological appearance of the cells in vitro was consistent with that of proerythroblasts, all cultures were tested for their ability to differentiate along the erythrocytic line under the influence of dimethylsulphoxide (DMSO). One of the cultures produced small quantities of haemoglobin independently of DMSO. Another was shown to produce haemoglobin, as well as to take up 59Fe and incorporate it into haem, only in the presence of DMSO. The 2 remaining cultures failed to produce haemoglobin, either spontaneously or in the presence of DMSO. Cells from each of the RV-induced cultures, when inoculated back into BDF1 mice, induced typical reticulum-cell sarcomas, without in vivo evidence of erythroid differentiation. In contrast, 2 morphologically identical but non-infectious cell lines derived from Friend virus-induced reticulum-cell sarcomas did not show erythroid differentiation in vivo or in vitro, either in the absence or presence of DMSO.