It has been shown that interleukin 4 (IL-4) stimulates the proliferation of cells from patients affected by adult T-cell leukaemia, the haematological malignancy aetiologically associated with human T-lymphotropic virus type I (HTLV-I). In the present study, human neonatal lymphocytes were exposed to HTLV-I in vitro in the presence of IL-4. The results showed that: (i) cultures exposed to HTLV-I in the presence of either IL-4 or IL-2 bound IL-4; (ii) IL-4 did not substitute for IL-2 as a growth factor in cell lines previously infected and maintained in IL-2; (iii) cultures exposed to HTLV-I and maintained in IL-4 or IL-2 became infected; and (iv) IL-4 sustained the growth of HTLV-I-infected cultures for a maximum of 14 weeks. Moreover, HTLV-I-infected cultures grown in IL-4 showed upregulation of the IL-4 message and lower expression of HLA-DR and CD25 when compared with counterpart cultures maintained in IL-2. These results suggest that continuous growth of T-lymphocytes induced in vitro by HTLV-I infection, at least temporarily, requires signals specifically provided by IL-2 and not by IL-4.