Although prostaglandin D2 is the most abundant prostanoid generated by human lung mast cells and causes bronchoconstriction in animals, its effects have not been studied in human beings. We have compared the effects of inhaled prostaglandin D2 and prostaglandin F2 alpha on specific airway conductance in seven normal subjects and seven patients with mild allergic asthma. In dose-response studies in normal subjects, prostaglandin D2 caused a significant (20 +/- 6 per cent) fall in specific airway conductance after the two highest concentrations (250 and 500 micrograms per milliliter), whereas prostaglandin F2 alpha had no effect. In the asthmatic subjects, both prostaglandin D2 and prostaglandin F2 alpha caused a dose-related fall in specific airway conductance, starting at the lowest concentration of 4 micrograms per milliliter. Prostaglandin D2 was 3.5 times more potent than prostaglandin F2 alpha. In a single-dose study of both drugs (250 micrograms per milliliter), a minor fall in specific airway conductance occurred with prostaglandin D2 in the normal subjects, and a larger fall occurred with both drugs in the asthmatic subjects. Maximum effects were seen at three minutes: there was a 75 +/- 5 per cent fall with prostaglandin D2 and a 33 +/- 8 per cent fall with prostaglandin F2 alpha. These results suggest that prostaglandin D2 may be involved in the pathogenesis of bronchoconstriction in allergic asthma.