A fusion protein (F-SU/GFP) which is comprised of the surface (SU) subunit of the Friend MuLV envelope glycoprotein and the green fluorescence protein (GFP) was generated by a baculovirus expression system. The F-SU/GFP specifically bound to mammalian tissue cultured cells expressing the mCAT-1, the receptor for ecotropic murine leukemia virus (ECO-MuLV). The expression level of mCAT-1 on hematopoietic cells was measured based on the capacity of cells to absorb the F-SU/GFP. In BALB/c mice susceptible to Eco-MuLV infection, all hematopoietic cell subpopulations absorbed the F-SU/GFP with the most prominent absorption observed in the TER119+ erythroblasts. Hematopoietic cells from C4W and AKR mice did not absorb the F-SU/GFP as readily as the BALB/c cells, probably due to expression of the envelope glycoproteins of endogenous Eco-MuLVs in these mice. Although AKR mice are susceptible to Eco-MuLV infection while C4W mice are resistant, these mice showed no apparent difference in the F-SU/GFP-absorbing capacity.