Previously, we reported a decrease of CD4+ T lymphocytes in workers exposed to benzidine and beta-naphthylamine, which are aromatic amines. In the present study, CD4+ T lymphocyte subpopulations (i.e., CD4+CD45RA+ and CD4+CD29+ cells), total CD4+ T lymphocytes, and CD8+ T lymphocytes were measured in 78 male dyestuff workers who had been exposed to aromatic amines. The aromatic amine workers consisted of 40 individuals who had engaged in production of aromatic amines (high-exposure group) and 38 workers who had handled only dyestuff in their job (low-exposure group). The control group consisted of 30 healthy male volunteers who were working at the same factory and who did not have a history of occupational exposure to aromatic amines or to other hazardous chemicals. The absolute and relative numbers of CD4+CD45RA+ (suppressor-inducer) T lymphocytes in the high-exposure group of aromatic amine workers were significantly lower than those in the control group, after controlling for the effects of age by the analysis of covariance. Similarly, the relative, but not absolute, number of total CD4+ T lymphocytes in the high-exposure group of aromatic amine workers was significantly lower than that found in the control group. It is suggested that the CD4+CD45RA+ T lymphocyte is a major site of the effects of aromatic amines on lymphocyte subpopulations.