Infection with FIV leads to an immune dysfunction characterized by a depletion of peripheral blood lymphocytes which express the feline homologue of CD4 (fCD4) (Ackley et al., 19906; Novotney et al., 1990). As the CD4 molecule is known to be the primary receptor for human immunodeficiency virus (HIV) (Dalgleish et al., 1984; Klatzmann et al., 1984), we have examined the role of fCD4 in infection with FIV.