For several decades bacteriologists and epidemiologists have been interested in the survival of micro-organisms in the airborne state. Studies by many investigators, recently summarized by Wells (1955), have produced data concerning certain aspects of bacterial survival or death, but the information has not generally sufficed to permit quantitative analysis when a fluid environment was changed in the airborne state.

1. Additional quantitative studies of the survival of micro-organisms in aerosols are reported.2. Substitution of nitrogen for oxygen in the ambient medium decreases k′2, the rate of secondary loss of viability ofM. candidus, Esch. coliandS. marcescens, by 30–50%.3. The rate of primary decay (k′1) ofEsch. colidoes not appear to be affected by a similar change of atmosphere.4. The rate of secondary decay, k′2, ofM. candidusappears to increase two-to three-fold for 10°C. increments in temperature.5. Treatment of suspensions ofEsch. coliwith petroleum ether alone or containing lipid material increases k′1in aerosols generated from such suspensions.6. Treatment ofMyco. phleiwith petroleum ether appears to lower resistance to subsequent sonic irradiation.These observations are consistent with the hypothesis that differences in the physical and chemical structure of micro-organisms are reflected in constants k′1, k′2and b.We gratefully acknowledge the effective interest of two former assistants, T. Grant Maple and Eugene Mills, as well as the careful, effective, and conscientious work of our technical assistants, Ann Barr, Nora Galins, Cynthia Hartwig, Grace C. Leigh, Elizabeth Neidhardt, Paulette Slaney and Martha Szerslip, who have at different times helped secure the data presented here. Mrs Joseph J. Ecker deserves special thanks for care in the preparation of the manuscript.