Two methods of interpreting histamine inhalation dose-response curves were compared in 27 normal and 41 asthmatic subjects. The histamine provocation concentration producing a 20% fall (PC20) in forced expiratory volume in one second (FEV1) was calculated on the basis of the lowest FEV1 after inhalation of saline and the lowest value after inhalation of histamine. The histamine threshold was determined as the first histamine concentration causing the FEV1 to fall more than 2 SD below the mean of five pre-histamine (three pre-saline, two post-saline) FEV1 determinations. The PC20 was on average one doubling concentration larger than the threshold. The PC20 provided better discrimination between asthmatic and normal subjects than did the histamine threshold and was significantly more reproducible. These findings suggest that the histamine threshold may prove useful for studies on populations, particularly those with a low degree of responsiveness to histamine, because of the possibility of measuring a response at a lower histamine concentration. On the other hand, the PC20 is preferable for clinical use in individuals because of its better discriminating power and better reproducibility.