Although it is thought that infection with human T-lymphotropic virus type I (HTLV-I) is immunosuppressive, this has not been clearly demonstrated among healthy carriers, and there are no data concerning delayed-type hypersensitivity (DTH). To evaluate this hypothesis, DTH to purified protein derivative (PPD) of tuberculin was measured in 126 healthy adults from an endemic area for HTLV-I infection in southern Japan. Among the 39 HTLV-I carriers, only 15% had detectable induration following PPD exposure, compared to 46% of the 87 non-carriers. In addition, the size of erythema among those carriers with a positive reaction was about 70% of that among non-carriers. Overall, there was a significantly inverse association between the degree of DTH response and prevalence of antibody. In relation to subjects with strong to moderate reaction, those with negative or indefinite reaction were 6 times more likely to be a carrier. This association was much stronger among subjects aged 60 years or older than among younger persons. These findings indicate that there is subclinical immunosuppression among HTLV-I carriers, which increases with age.