Chlorine and bromine mixtures were tested for killing bacteria in different types of water. Bromine worked better in water with lots of nitrogen, while chlorine was better in water with less nitrogen. Mixtures of chlorine and bromine were most effective when there was less hypobromite. Chlorine-resistant bacteria were found in water treated with chlorine, but they were sensitive to low levels of bromine when mixed with chlorine. Some of these bacteria changed into slower-growing organisms.