We have examined in vitro cell-mediated lymphocyte responses to Concanavalin A, (Con. A), and the effects of histamine and indomethacin upon these responses, in normal subjects, and patients with extrinsic and intrinsic asthma, and chronic bronchitis. Lymphocytes from both intrinsic and extrinsic asthmatics are particularly sensitive to histamine-induced suppression of their response to Con. A, and this increased sensitivity was reversed by indomethacin. In these respects, lymphocytes from intrinsic and extrinsic asthmatics behave in an identical fashion, but differ significantly from lymphocytes from both normal subjects and patients with fixed airways obstruction (chronic bronchitis). It is suggested that there is a common immunological mechanism in extrinsic asthma and intrinsic asthma.