Antibody‐dependent cellular cytotoxicity (ADCC) and natural killer (NK) ativity were examined using MT‐2 cells persistently infected by HTLV‐1 as target cells, and mononuclear cells as effector cells, from helathy one‐week‐old newborn babies, infants, children and adults. More than 10% of ADCC was observed in 17 newborn babies out of 22 (77.3%) and in all 67 healthy one‐month‐old babies to adults, by adding serum from anti‐HTLV‐1 positive carriers. When anti‐HTLV‐1 negative serum was added, less than 10% of ADCC was observed. If infants without anti‐HTLV‐1 antibodies were breast‐fed they had the possibility of HTLV‐1 vertical transmission. There was no significant decrease in NK activity between 90 healthy newborn babies, infants, children, or adults. These results suggest that ADCC and NK activity protect against the transmission o