The reticuloendotheliosis viruses (REV) are a family of highly related retroviruses isolated from gallinaceous birds. On the basis of sequence comparison and overall genome organization, these viruses are more similar to the mammalian type C retroviruses than to the avian sarcoma/leukemia viruses. The envelope of a member of the REV family, spleen necrosis virus (SNV), is about 50% identical in amino acid sequence to the envelope of the type D simian retroviruses. Although SNV does not productively infect primate or murine cells, the receptor for SNV is present on a variety of human and murine cells. Moreover, interference assays show that the receptor for SNV is the same as the receptor for the type D simian retroviruses. We propose that adaptation of a mammalian type C virus to an avian host provided the REV progenitor.