Resistant cells of Pseudomonas aeruginosa ATCC 9027 which were grown in the presence of 1 mg of benzalkonium chloride (BC) per ml caused only a mild conjunctivitis when they were dropped onto the scratched corneas of rabbits. In contrast, cells of the BC-sensitive parent strain induced a severe keratoconjunctivitis. In addition, the BC-grown cells also had a reduced capacity to produce kidney infections in mice as compared to the parent strain. BC-grown cells acted as weak complex antigens which conferred slight protection against lethal doses of BC-grown cells. No cross-protection to cells of the parent strain occurred. The data indicate that growth in the presence of BC results in cells with reduced virulence.