Cats in the primary stage of Feline Immunodeficiency Virus (FIV) infection and FIV seronegative cats were vaccinated with a modified live Feline Panleukopenia virus vaccine (FPV-MLV). The FPV-MLV strain was not pathogenic for FIV seronegative cats, whereas in FIV infected cats it produced severe clinical signs, similar to those typically observed in cats naturally infected with field strains of FPV (fever, diarrhoea, leukopenia).