Single oral doses (5, 10 and 15 mg) of the new H1-receptor antagonist 3,7-dihydro-7-[2-hydroxy-3-[4-[3-phenylthio) propyl]-1-piperazinyl]propyl-1,3-dimethyl-1H-purine-2,6-dione dihydrochloride (tazifylline, RS-49014) were administered at 7-day intervals to 12 atopic and 12 non-atopic volunteers in a double-blind randomised cross-over study. Antiallergic activity was evaluated from pre to 60 min post dose inhibitions of wheal and flare areas provoked by various cutaneous challenges. Atopic subjects showed greater skin reactivity than non-atopics to some challenges. Tazifylline was associated (in atopics and non-atopics) with statistically significant dose related inhibitions, of flare over 5-15 mg and of wheal (10-15 mg) induced by the more potent and reproducible challenges of codeine, 1% histamine and anti-IgE. Antiallergic activity of tazifylline in the 10-15 mg dose range was superior to 5 mg without the intervention of clinically significant sedative effects at any of the 3 dose levels tested.