Four patients with pigeon breeder's lung and positive lymphocyte transformation tests to pigeon antigen were given 250 mg hydrocortisone intravenously, after which the test was repeated at 6, 24, and 30 hr. A significant reduction in lymphocyte responsiveness to pigeon antigen was evident 6 hr after the steroid dose, which reversed within 24 hr after the treatment. This was associated with transient ‘false negative’ lymphocyte transformation tests in two of the four patients, so this effect of steroid treatment may influence the reliability of this test, if used in the diagnosis of patients with allergic alveolitis. Phytohaemagglutinin responsiveness was not affected by the hydrocortisone treatment. This may indicate differences between the cell populations that respond, after steroid treatment, to antigen and to mitogenic stimulation.