In a double-blind crossover placebo controlled study, the inhibitory effects of three different doses of nedocromil sodium on the immediate bronchoconstrictor response to antigen challenge were compared with a placebo. All the test treatments were inhaled three hours before challenge. Four asthmatic patients took part in the study and the test treatments were taken in a randomized order. All three doses of nedocromil sodium significantly reduced the bronchoconstrictor response and protected the patients against antigen challenge, compared with placebo. The highest dose of 2 mg was the most effective dose. All test doses of nedocromil sodium were safe and well tolerated by the patients.