SUMMARY A crude preparation of pulmonary surfactant was obtained from rabbit lungs by lavage of the broncho-alveolar space. Lyophilized and reconstituted solutions of surfactant were incubated with various pure strains of bacteria. Some bacteria reduced surface tension during incubation, these were P. aeruginosa, A. aerogenes, Proteus species, K. Pneumoniae. and to a lesser extent D. pneumoniae. Gram positive bacteria did not appear to alter the surface tension of the pulmonary surfactant solutions; these included S. aureus. S. epidermidis, S. pyogenes (Lancefield Group A), S. viridans, and S. fecalis.