The effect of UK-74,505, a specific platelet-activating factor (PAF) antagonist, on the early (EAR) and late asthmatic response (LAR) to inhaled allergen was studied in a randomized, double-blind, placebo-controlled crossover study. A total of eight adult male atopic asthmatic subjects completed the protocol (one withdrew after screening), all having demonstrated a dual response to inhaled allergen (EAR, > 20% fall in FEV1 between 0 and 1 h; LAR, > 15% fall in FEV1 between 4 and 8 h after challenge). Subjects were studied on 2 days at least 10 days apart. After measurement of baseline FEV1, subjects ingested a single oral dose of 100 mg UK-74,505 or matched placebo (P). Allergen challenge was performed 3 h later and the FEV1 was then measured for 8 h. There was no difference between UK-74,505 and placebo in the maximum percentage change from baseline (+/- SEM) for either EAR or LAR (EAR, UK-74,505 -25.6 +/- 4.8%, P -24.0 +/- 3.3%; LAR, UK-74,505 -20.8 +/- 4.4%, P -25.7 +/- 3.8%). There was no significant difference in the area under the percentage change from baseline FEV1-time curve. Ex vivo platelet aggregation to PAF was measured at 0, 2, 6, 8, and 10 h after the dose. There was marked inhibition of platelet aggregation to PAF for 10 h following UK-74,505 but not placebo (% maximum aggregation to PAF, UK-74,505, -69.9%; P, 0.13%; p = 0.0001). Histamine challenge was performed in five patients the day before and after each study day. There was no significant difference between UK-74,505 and placebo in PD20 to histamine (mean PD20 before and after UK-74,505, 1.31 and 0.96 mumol; P, 1.32 and 1.17 mumol). UK-74,505 did not affect either the EAR or the LAR to inhaled allergen or bronchial responsiveness, despite its potency and long duration of action. This suggests that PAF does not have a major role in the acute response to inhaled allergen.