Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) is a lymphotropic retrovirus. Cells infected with HTLV-1 in vitro, when maintained in interleukin-2 (IL-2) can be immortalized, remaining for a long time strictly dependent on IL-2 addition. In this study we have compared the effect of interleukin-4 (IL-4) and interleukin-2 on HTLV-1 infection of cord blood or normal adult mononuclear cells. The results showed that either cultures of cord blood or normal adult T cells are susceptible to HTLV-1 infection in presence of IL-4 as well as IL-2. Moreover HTLV-1 infected cells in the presence of IL-4 survived only for a limited length of time in culture, while those grown in IL-2 showed the characteristics of immortalized cell lines. Moreover the profile of cytokine production showed a different pattern in HTLV-1 infected cell lines maintained in IL-4 or IL-2. This suggests that the lymphokines differently modulate retrovirus infection in vitro.