An inactivated whole-virus FeLV vaccine, developed from a molecularly cloned FeLV isolate (FeLV-61E-A), was assessed for its ability to protect cats against homologous and heterologous virulent viral challenge. The fractions of cats that resisted the induction of persistent viremia after FeLV challenge were as follows: FeLV-61E-A vaccine, 95%; adjuvant controls, 26%; and established commercial control FeLV vaccine, 35%. The prechallenge mean neutralizing antibody titers for each group were as follows: FeLV-61E-A vaccine, 1:43; adjuvant controls, < 1:8; and commercial control FeLV vaccine, 1:12. The prototype FeLV-61E-A vaccine was developed commercially for immunization of pet cats by substitution of a proprietary adjuvant and development of stable, high antigen production cell lines. This vaccine (Fel-O-Vax) has been studied extensively, alone and in multivalent combination with other feline virus vaccines, in seven efficacy trials involving a total of 150 immunized cats. These studies yielded an FeLV-resistant fraction of 87% in vaccinated cats as compared with 8% in adjuvant controls. The duration of immunity induced by an FeLV-61E-A commercial vaccine (Fel-O-Vax-LvK IV) was also assessed. One year after vaccination, 100% of challenged vaccinated cats and none of challenged controls resisted induction of persistent viremia. The results of these studies demonstrate that an inactivated FeLV vaccine prepared from a molecularly cloned subgroup A FeLV produces a high level of protective immunity against heterologous and homologous FeLV infection. This vaccine-induced immunity is durable for at least 1 year without intervening booster immunization or exposure to virus.