PGF2SaL challenge tests were performed in twenty-three patients with bronchial asthma and six patients with chronic bronchitis; two patients with allergic rhinitis and four healthy volunteers also participated in the trial. FEV1 measurements were used in assessing the reactions. PGE2α challenge proved safe and practical in clinical use. A wide scattering in PGF2α reactivity was seen, but sensitivity to PGF2α did not correlate to several clinical criteria. Comparison with an ordinary histamine challenge revealed that the sensitivity of the patients to PGF2α and histamine was not uniform.