The types of receptors on the surfaces of human and other mammalian cells for 13 C-type and 7 D-type retrovirus strains were determined by interference to the formation of syncytia and the plating of viral pseudotypes. All the D-type simian retroviruses (SRV-1-5, SMRV, PO-1-Lu) share a common receptor which is also utilized by the baboon and cat endogenous C-type viruses (BaEV, RD114). Syncytial cross-interference was also observed in human cells between the gibbon ape leukemia/simian sarcoma associated viruses (GALV/SSAV) and feline leukemia virus subgroup B (FeLV-B). Amphotropic and xenotropic murine leukemia viruses (MLV-A, MLV-X), bovine leukemia virus (BLV), and FeLV-C infect human cells via unique cell surface receptors. Human T-cell leukemia viruses types 1 and 2 (HTLV-1, HTLV-2) share a common receptor with related chimpanzee and simian viruses (STLV). Thus seven distinct receptor groups were delineated on human cells for C-type and D-type retroviruses.