Guinea-pigs were depleted of circulating polymorphonuclear leucocytes (PMN) by administration of anti-polymorph serum. Groups of animals were then infected by aerosols containing different doses of Legionella pneumophila and the effects compared with those in intact infected controls. Elimination of PMN lowered the dose of L. pneumophila necessary to establish infection, increased bacterial numbers in the lungs and caused much higher mortality. It did not change the nature or extent of pulmonary lesions. The findings confirm the importance of PMN in defence of the lung against L. pneumophila infection and indicate that PMN and their enzymes are not responsible for the pulmonary lesions, which are probably caused directly by the bacteria.