The purpose of this study was to describe the pattern of the ocular late phase reaction in patients with ocular hay fever. The authors monitored the clinical and cytological conjunctival response after topical allergen challenge. A conjunctival provocation test was performed in six ryegrass-sensitive patients and six healthy volunteers with 320,000 Biological Units (Pharmacia, Uppsala) of ryegrass antigen, diluted in albumin, in one eye; albumin diluent alone was used in the second (control) eye. Clinical symptoms were evaluated before challenge after 20 min, then hourly for 12 hours and 24 hours after challenge. Cell counts in the tear fluid were performed at the same time interval The results obtained show a significant clinical reaction during the entire period, differing in peaks in individual cases. This reaction was associated with a persistent inflammation, with neutrophils in the early phase and eosinophils and lymphocytes in the late phase period The data thus show that ocular challenge with high allergen doses induces clinical and cellular evidence of an ocular late phase reaction and suggest that the conjunctival provocation test may be used for a better understanding of the pathogenesis of mild but persistent ocular inflammation in ocular hay fever.