Materials and Methods (1) Passively sensitized human lung challenged by antigen in the presence of human bronchial strip. Samples (400 mg) of human lung parenchyma, cut as a single long strip about 2 mm thick, were passively sensitized by incubation overnight at room temperature, followed by 1h at 37 C, with a 1: 40 dilution of human allergic (Stopford) serum from a patient sen sitive to pollen antigen. Unsensitized human bronchial tissue from the same lung was stored overnight at 4 C in oxygenated tyrode solution. On the following morning, an isolated bronchial strip was prepared and set up in a 15 ml organ bath, as described by SHEARD [1968J. The sensitivity of the preparation was tested by the addition of histamine (2-4/ig/ml) to the bath. All washing was performed