Corneal and conjunctival biopsies of 13 patients with rheumatoid arthritis and corneal ulceration (RA-keratomalacia) have been characterized by immunohistological and histochemical analysis. Biopsies from 13 patients with bacterial conjunctivitis, 7 patients with allergic conjunctivitis, 15 patients with senile cataract and 15 patients with keratokonus served as controls. The phenotypic composition of the conjunctival inflammatory infiltration of rheumatoid corneal ulceration was not significantly different from the other inflammatory eye diseases studied. However, conjunctival epithelial cells of all RA-patients showed strong de novo expression of HLA-DR- and DP-antigens. HLA-DQ-antigens were only weakly expressed in a minority of patients. In bacterial conjunctivitis a less intense HLA-class-II-expression was found to be restricted to HLA-DR-antigens. Furthermore, in RA patients stromal fibroblasts of the cornea expressed lysosomal elastase. Both observations could be explained by paracrine action of interleukins produced by infiltrating T-lymphocytes and macrophages. Thus, it might be tempting to speculate that immunologically induced, elastase mediated autodegradation of corneal stroma may be an important factor in the pathogenesis of rheumatoid corneal ulceration.