Contraction of passively sensitized human bronchus induced by antigen challenge in vitro, was reduced by the prostaglandin F2alpha synthetase inhibitor, indomethacin. 2. Prostaglandin F2alpha was released during challenge and this release was inhibited by indomethacin. 3. There was a significant correlation between prostaglandin F2alpha release and antigen challenge-induced contraction, suggesting that this substance may contribute to the bronchoconstrictor response of sensitized human bronchus to antigen.