Cetirizine is a new selective H1-antagonist. In a double-blind and placebo-controlled study, 11 subjects underwent a conjunctival provocation test (CPT) with a grass pollen extract to determine the effect of cetirizine on allergen-induced itching and redness of the eye. Results of CPTs were reproducible, as no difference was observed between the CPTs performed without any pre-treatment and the CPTs after placebo. Compared to placebo, cetirizine administered orally (10 mg twice a day for 4 days) and in double-blind fashion increased the conjunctival reaction threshold (P less than 0.004) by inhibiting both redness and itching of the eye. Cetirizine should protect patients against allergic conjunctivitis.