Bronchial responsiveness to methacholine (PD20 FEV1) was assessed in 22 asthmatic subjects approximately 5 days prior to bronchoalveolar lavage (BAL). A PD20 FFV1 could not be attained in 20 matched controls with normal pulmonary function. BAL was performed in all subjects, 3 ± 60 ml aliquots of buffered saline being Introduced into a segment of the middle lobe and immediately aspirated into siliconized glassware at 4°C. After filtration, cells were counted, and the cell pellet resuspended in medium 199. Cytospin slides were prepared and a differential cell count performed. Lymphocyte subsets were identified by labelling further cytospin preparation with specific monoclonal antibodies (Leu series) against T3, T4, T8 and B cell markers, followed by a fluorescent antibody marker. The slides were coded, and 100 lymphocytes were then randomly scanned for fluorescence on each cytospin preparation. The median total lymphocyte counts were significantly greater in the asthmatic subjects, but this increase was confined to the T cell subgroups. The mean T4/T8 ratio was similar in asthmatic (1.51) and control (1.45) subjects. Log PD20 FFV1 correlated positively with total lymphocyte counts (r= 0.42, P < 0.05), and with total T8 counts (r= 0.60, P < 0.05). Correlations between bronchial responsiveness and T3, T4 and B lymphocyte numbers all failed to reach significance, and there was no correlation with T4/T8 ratios. The increase in BAL lymphocyte counts in asthma appears to be due loan absolute increase in T cell subsets, especially the T8 lymphocyte subgroup, and is most marked in mild asthmatics.