The GM-CFC assay for granulocyte-macrophage progenitors and the BFU-E and CFU-E assay for early and late erythroid progenitors from cat bone marrow were characterized. GM-CFC gave 59 +/- 4 to 118 +/- 6 colonies per 10(5) bone marrow cells using colony stimulating factors (CSF) from cat, mouse or human sources. The CFU-E and BFU-E assays gave 114 +/- 7 and 58 +/- 7 colonies respectively with optimum doses of erythropoietin. Irradiated cat bone marrow cells were good sources of CSF and of burst promoting activity for these assays. Kittens infected with feline leukaemia virus, subgroup C (FeLV-C), which induces pure red cell hypoplasia, showed the incidence of BFU-E decreased to 25-35% of controls as early as one week postinfection, and even lower values at later times. In contrast, the incidence of GM-CFC remained normal for several weeks. No evidence of inhibitory cells or of lack of stimulatory cells in the infected marrows was seen when they were cultured together with normal marrow in the BFU-E assay. Conversely, normal marrow cells were not able to restore BFU-E growth from infected marrow. This suggests a direct action of FeLV-C on early erythroid precursors. Infection with FeLV, subgroup A, which induces only a mild transitory anaemia, produces only a moderate decrease in the incidence of BFU-E.