SummaryIn a previous study on immune responsiveness among asymptomatic human T-cell leukemia virus type I (HTLV-I) carriers, we found that carriers had significantly reduced delayed-type hypersensitivity (DTH) response to purified protein derivative (PPD) skin testing. The association was strongest among persons at least 60 years of age. In order to evaluate this finding further, we evaluated the response to both PPD and phytohemagglutinin (PHA) in an elderly population. Fifty-six consecutive hospitalized patients with nonimmunosuppressive diseases were examined. None had a history of tuberculosis nor evidence of the known HTLV-I-associated diseases. The subjects' ages ranged from 62–93 years (median = 75 years); 43 were women and 13 were men. Twenty-two of the subjects were HTLV-I antibody positive. Among the carriers, there was an increased level of nonreactivity to PPD, the relative risk adjusted for age (RR) being 1.9 (95% confidence interval, 0.62–5.8), as well as to PHA of RR = 2.3 (0.60–9.0). When subjects were cross-classified for response to both skin tests, 15 of 17 carriers were nonreactive to either or both antigens compared to 15 of 25 noncarriers [RR = 5.1 (0.99–25.9) (pvalue, one-sided = 0.026)]. The decline in reactivity to both antigens increased with age, but was consistently lower among the carriers. Among subjects with positive reactions to PPD, the degree of reaction as measured by the size of erythema was reduced among the carriers; however, for PHA responders, the response in carriers appeared to be normal. Among the HTLV-I antibody negative subjects, the size of erythema for both antigens was strongly correlated (p= 0.01). Among the carriers, there was no correlation (p= 0.92). These findings confirm our earlier report of reduced DTH response in older HTLV-I carriers and extends it to nonspecific mitogen challenge, suggesting that the defect in triggering DTH is not related to antigen-specific memory. However, once triggered, the level of erythema to the two antigens is not correlated in carriers.