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. fCD4 expression in the cat appears to be restricted to a sub-population of peripheral T cells and their thymic precursors (Ackley et al., 1990a) and does not correlate with susceptibility to infection either in vivo (English et al., 1993) or in vitro (Willett et al., 1991; Hosie et al., 1993)