SUMMARY : Because of adsorption phenomena, copper sulphate solutions containing traces of protein are less toxic to bacteria in droplets than to similar bacteria in large volumes. With dilute phenol solutions the reverse was found. The quantitative results suggest that the non-flagellate organism studied moves with about 25 times the energy of Brownian motion.Although much information is available about the killing of bacteria by different chemicals, almost all of it has been obtained from investigations that used volumes of suspensions of the order of 1 ml. or more. Little has been published about the rate of killing of bacteria suspended in droplets owing, for the most part, to the difficulty of adapting the standard viability counting methods for use with such small samples. However, the urea method of measuring bacterial viability (Valentine & Bradfield, 1954) does allow the yo live organisms in small drops to be determined. In outline this method consists of incubating a drop of the suspension under investigation on the film of an electron microscope specimen support placed on a nutrient agar medium containing 3% (w/v) urea. The urea does not stop the growth of any live bacteria in the drop but it does inhibit their division and each develops into a long filamentous form. The dead bacteria remain unaltered in appearance. After about 3 hr. of incubation, the film is fixed in formalin vapour, washed and examined in the electron microscope. A determination of the ratio of long to normal-length organisms then gives a reliable estimate of the ratio of live to dead bacteria originally in the drop. In the experiments now described, this urea method has been used to measure the rate of killing of bacteria by dilute copper sulphate and phenol solutions in drops 0.6 mm. in diameter. Observation of bacteria in drops of this size with the light microscope showed that when the surface tension of the suspending fluid was about that of water, the bacteria arriving at the surface of the drop were held there by surface forces and unable to return to the interior. Within a few minutes, many of the bacteria in a small drop have gathered and remain on the surface and it is here that much of the interaction between them and any chemicals in the drop takes place. Adsorption phenomena may cause the chemical composition of this surface layer to differ from that of the interior and thus it is quite possible that the death-rate of bacteria in droplets containing toxic substances may be entirely different from that in larger volumes of the same suspension. This point is in fact demonstrated by the following experiments.