Animals infected with conventional anaemia (FVA) or polycythemia-inducing (FVP) strains of the Friend virus develop lethal erythroleukaemia. A variant strain, RFV, induces an initially identical disease except that it spontaneously regresses in 50% of infected mice. To determine whether pluripotent stem cells as measured by spleen colony forming units (CFU-s) in leukaemic mice are productively infected with virus and whether their infection influences the outcome of the disease, we tested CFU-s from leukaemic mice for susceptibility to cytotoxicity by monospecific antiviral gp70 antiserum. Spleen CFU-s from progressively leukaemic (FVP, FVA and RFV) mice were productively infected with virus. CFU-s in RFV progressors became infected by 40 days post-virus inoculation. FVA and FVP progressors became infected between 15 and 21 days post virus. Infection of CFU-s was accompanied by an increase in the proportion of replicating (S phase) CFU-s in these populations. Spleen CFU-s from fully regressed RFV regressor mice were uninfected and remained so throughout the course of their disease. Bone marrow CFU-s in both regressors and progressors remained uninfected and were not induced to increased cell cycling.