The efficacy of procaterol, a new selective beta 2-stimulant (by pressurized aerosol), and its protection against the effects of non-specific bronchial stimulation (fog test) was evaluated in ten hyperreactive seasonal atopics in an open trial of twelve days duration. Preliminary results had proved that single (10 micrograms) and repetitive (10 micrograms t.i.d.) doses of procaterol induced a strong bronchodilation. Accordingly the pre-medication with procaterol (10 micrograms) completely prevents the flow limitation subsequent to the non-specific bronchial stimulation.