A group of patients with vernal disease was investigated for serum IgE levels, specific antibody to external allergens in serum and tears, and the frequencies of the HL-A antigens. Biopsies taken from the conjunctiva were examined by means of light and electron microscopy for the presence of mast cells and other inflammatory cells. Serum IgE was elevated in 75 per cent of the patients, all of whom demonstrated positive prick tests to common allergens and specific antibody in their serum using the radio-allergosorbent test (RAST). Specific antibody was also demonstrated in tears using the same technique. The distribution of the HL-A antigens showed no difference in frequency when compared with controls. Conjunctival biopsies demonstrated numerous mast cells, the more superficial of which were degranulating in active disease. In control biopsy material, mast cells were densely packed with granules. The clinical manifestations in vernal disease together with the infiltration of the tarsal conjunctiva with many types of inflammatory cell, are possible caused by mediators which are known to be produced by degranulating mast cells when they are stimulated by allergens.