The efficacy of interferons (IFNs), used empirically to treat retrovirus-infected cats has been shown in vivo, but the direct effect on infected cells is largely unknown. Ten-fold serial dilutions of three recombinant IFNs available for therapy, human IFNalpha(2a), IFNalpha(A/D) and feline IFNomega were added to the chronically FeLV-infected cell line FL74. IFNs did not apparently affect viral protein expression. However, reverse transcriptase activity (RT), directly proportional to the amount of infectious free virions, decreased with increasing concentrations of IFN and longer treatment times. The induction of apoptosis by IFN was suspected. Results of its evaluation by annexin V-Fluos staining showed that IFNs decreased the viability of treated FeLV-infected cells, and increased apoptosis, but not of non-infected cells. According to the IC(50), rHuIFNalpha(A/D) appeared to be the most effective IFN in inhibiting RT.