The biological activity of various cigarette smokes has been assessed by treating cell cultures of rabbit alveolar macrophages with salt solutions of "whole smoke," smoke "vapor phase," and smoke "particulate phase." The results of these tests revealed degrees of toxicity that varied in accordance with the smoke treatment used. Further, these differences in toxicity were statistically significant at the 95% confidence limit. Observations of changes in cell morphology on exposure to the smoke solutions indicated smoke toxicity to be caused, in part, interference with cell membrane activity.