THE 'single-step' method devised by Ellis and Delbruck1 has proved of immense value in the hands of a number of workers in studying the multiplication of various bacteriophages ; but it was devised specifically to investigate the behaviour of bacteria and adsorbed phage at high dilution, and gives no direct information about the more complex situation normally occurring, when phage released by lysis of one bacterium can interact further with other bacteria. Since this information was necessary for a line of investigation we are pursuing, and since, moreover, it might prove of more general interest, we have devised the method described below.