The immune response to the Legionnaires' disease (LD) bacterium has been shown serologically. Recent evidence suggests significant differences in seroreactivity depending on the source of antigen; these data now unequivocally show strain variation for the LD bacterium. Nothing is known about the cellular immune reactivity in patients with Legionnaires' disease. There is little evidence that the lesions and manifestations of the disease are due to immunopathologic mechanisms. Possible explanations for the pathogenesis of pulmonary lesions in Legionnaires' disease include toxic factors, bacterial chemotactic factors, neutral proteases (with complement cleaving activity), lipopolysaccharides, and products of lymphocytes.