Feline leukaemia virus (FeLV) is a major cause of degenerative and proliferative diseases of myeloid, lymphoid and erythroid origin in domestic cats. The envelope of FeLV, which is a typical retrovirus, is studded with a transmembrane protein (TM) that anchors an external surface glycoprotein (SU). The FeLV SU (gp70) carries both neutralising epitopes and subgroup phenotypic determinants. FeLV occurs naturally in 3 subgroups, which are defined by a superinfection interference assay, suggesting that three FeLV cellular receptors exist, one for each subgroup. The regions of SU which determine the A/C phenotype have been defined as lying in a small variable region towards the amino terminus of the protein.