Summary Under the conditions of this study it appears that there is a “toxic” effect of smoke gases on strain L cells in culture. This results both in morphological changes in the cells and in alterations in growth curves as indicated by the reduction of cell populations. The smoke from the combustion of either cigarette paper or onion skin paper appears to be more “toxic” than that from the combustion of a cigarette or cigarette tobacco alone. The effect of cigarette or other smoke gases may result in part, at least, from the particulate matter present in smoke gases.