Cell populations obtained by bronchial lavage from human subjects were examined for the presence of cells related to the mast cell-basophil series. Such bronchial lumen histamine-containing cells (BLHCC) were identified. The BLHCC stained with toluidine blue may be identified by bright field or dark field microscopy. The BLHCC are alive as evidenced by ability to release histamine (H) after exposure to anti-IgE or calcium ionophore. Although H release from peripheral blood leukocytes by these two agents is potentiated by the presence of D2O, H release from BLHCC of the same subjects by anti-IgE or calcium ionophore was not potentiated by D2O. In studies comparing bronchial cell populations of humans and rhesus monkeys with peripheral blood leukocyte populations of the same subjects, the histamine content of the bronchial cell population was much higher in rhesus monkeys. IgE/Alb ratios of respiratory secretions and serum of the same human subjects were of the same order of magnitude in contrast to previous comparisons done on these fluids in rhesus monkeys.