Abstract
Feline leukaemia viruses of subgroups B and C multiply in human and canine cells, while subgroup A viruses do not. This host range restriction is determined by the virus envelope and operates at the level of virus entry into the cell. Subgroup A virus genomes are expressed and replicated when they are introduced within B subgroup envelopes into human or dog cells. Therefore, since they are phenotypic mixtures of A and B subgroup viruses, the majority of feline leukaemia virus isolates will infect human and canine cells with the subsequent production of FeLV of each subgroup.