Levocabastine is a new H1 receptor blocking antihistamine which is intended for topical use in the treatment of allergic conjunctivitis. The protective effect of the drug in conjunctival provocation test (CPT) was evaluated in a double blind study of 25 children aged 9-17 years with confirmed pollen allergy. One drop of levocabastine, cromoglycate or placebo was instilled into the conjunctival sac of both eyes. After 15 min CPT was performed, starting with 320 BU of pollen extract. The allergen dose was increased every 10 min in half 10-log steps in the right eye until a positive reaction occurred, or the top dose 320,000 BU was reached. The lowest dose resulting in a positive CPT reaction, i.e. at least 50% of the conjunctiva with erythema, was defined as the allergenic threshold dose (ATD). Pretreatment with levocabastine resulted in a median ATD of 32,000 BU, compared with 10,000 after cromoglycate (P less than 0.001) or placebo (P less than 0.01). Levocabastine was also superior in reducing subjective itch in the eyes. Determination of the ATD can be used as a relatively quick assessment of drugs intended for the treatment of allergic conjunctivitis.