The product encoded by the latent membrane protein (LMP) gene of Epstein-Barr virus (EBV) has been implicated as a transforming protein by a number of studies. We have examined the effects of LMP expression in FDCP-mix cells, a growth factor-dependent multipotential murine 'stem cell' line. Our studies show that LMP reduces the generation of clonogenic cells and leads to the production of cells expressing a marker (lysozyme M) characteristic of mature monocytes and macrophages. Furthermore, cells expressing LMP are compromised in their ability to produce mature neutrophils. These data suggest that expression of LMP in primitive cells can modulate their self-renewal and differentiation potential and provide evidence in support of the suggestion that EBV may be involved in some of the maturation defects of haemopoiesis.