1. The adsorption rate constant of phage to bacterium is found to change between wide limits, depending on the physiological state of the bacterium.
2. The experiments of Krueger and of Schlesinger on the residual free phage in contact with an excess of bacteria are discussed and the view of Schlesinger, that they represent phage particles with reduced affinity to the bacterial host is supported by experiments.
3. The theory of von Smoluchowski and Schlesinger is compared with the experiments.
4. The implications of these findings for the assay methods currently used are discussed.