The lymphocyte subsets in the bronchoalveolar lavage fluid (BALF) and the peripheral blood of 25 healthy volunteers were examined by analysis with a fluorescence-activated cell sorter. Comparison of the lymphocyte subsets in the BALF with those of the peripheral blood revealed much higher values for the ratios of each Leu 3a+ (CD4), Leu 3+8-, and Leu 2+15- cells, while the ratios of Leu 1+ (CD5), Leu 2a+ (CD8), Leu 7+, Leu 8+, Leu 10+, Leu 11a+ (CD16), Leu 12+, and Leu 2+15+ cells were low in the BALF. The above results indicate that the lymphocyte subsets in the BALF from healthy individuals are mainly composed of cells with surface phenotypes of helper T cells and cytotoxic T cells with virtual absence of cells carrying suppressor T and NK cell phenotypes, and with low B cell ratio. Therefore, it is assumed that the local immune mechanism of the lung is different from that of the peripheral blood.