Twelve asthmatics with dual bronchial and skin reactions after allergen challenge received topical treatment with a 5%-indomethacin cream half an hour before and up to 7 h after intradermal allergen and histamine injections. The erythema during the first 20 min of the wheal and flare reaction (WFR) was not affected, neither were the diameters of wheals and flares. 40 to 60 min after injection we observed a marked reduction of the erythema in histamine- and allergen-tested skin areas of 10 patients. This effect lasted up to the 5th h after injection of high allergen doses. During the fully developed late cutaneous reactions (LCR) no effect of indomethacin on the erythema was observed, the edema of LCR was only insignificantly reduced. These results suggest that the erythema in LCR between the 1st and 4th h is caused, at least in part, by local formation of prostaglandins.