Rabbits, pre-immunized by intravenous inoculations of live, or of freeze-killed microfilariae of Onchocerca volvulus, were later challenged by subconjunctival inoculation of live microfilariae. Those pre-immunized with live microfilariae showed a marked chemotic conjunctivitis and reactions in the cornea (stromal keratitis and limbal abscesses), starting within one day of challenge. They were classed as sensitized. Those pre-immunized with dead microfilariae produced minimal reactions also starting on day 1, and were classed as tolerant. Histologically the reactions were distinctly greater in sensitized than in tolerant animals. The predominant cells in the inflammatory exudate were polymorphonuclear leucocytes and lymphocytes. The immunological basis for the differences between sensitized and tolerant rabbits is discussed, together with its possible bearing on human onchocerciasis. The effects of long-term subconjunctival and sclerocorneal inoculation of microfilariae were compared in one sensitized, one tolerant, one previously exposed, and one sensitized control rabbit. The sensitized test animal showed lesions resembling the sclerosing keratitis of human onchocerciasis. Immature microfilariae taken from intra-nodular fluid showed little ability to penetrate the cornea of the rabbit, and were minimally pathogenic.