Using a sensitive single isotope enzymatic assay we measured bronchoalveolar lavage (BAL) fluid histamine in asymptomatic normal (nonallergic), allergic rhinitic, and allergic asthmatic subjects. Normal subjects were found to have little or no detectable amounts of histamine in BAL fluid (11 +/- 11 pg/ml), and few BAL fluid mast cells. In comparison, the allergic rhinitics and allergic asthmatics had much higher amounts of BAL fluid histamine (113 +/- 53 and 188 +/- 42 pg/ml, respectively), and a significantly greater number of BAL fluid mast cells. Furthermore, despite having equivalent baseline pulmonary function values, allergic asthmatics with BAL fluid histamine levels greater than 100 pg/ml required only 7 +/- 2 breath units of methacholine to induce a 20% drop in forced expiratory volume in 1 s (FEV1) (PD20FEV1) while asthmatics with BAL fluid histamine levels less than 100 pg/ml required 49 +/- 19 breath units (P less than 0.05). These data suggest that allergic asthmatics have ongoing lung mast cell degranulation that might contribute to the etiology of airway hyperresponsiveness.