We have examined the activities of Epstein-Barr virus (EBV)-specific memory T-cells of carriers of adult T-cell leukemia (ATL) associated retrovirus (HTLV-I) and non-carriers in an ATL-endemic area, all of whom were EBV-seropositive, by assay of EBV-induced B-cell focus regression. The result showed that immune suppression, as represented by lowering of the regression, was present in 18 (29%) out of 63 healthy HTLV-I carriers, in contrast to none of 63 matched control persons (healthy non-carriers). This finding suggests that a suppression of cell-mediated immunity is induced by a persistent infection with ATL retrovirus.