Several methods are available for testing the sensitivity of organisms to the sulphonamide drugs (Harper and Cawston, 1945; Evans, 1948; Kokko, 1947), but none of them is entirely suitable for clinical use. The main difficu'ty in using a test similar to that for the antibiotics is the presence of sulphonamide-antagonizing substances in routine media. Such substances were first found in peptone (Lockwood, 1938): it was shown that their effect could be counteracted by human blood, heart broth, and serum (Lockwood, 1938; Stamp, 1939). Two lines of approach have therefore been followed in developing sulphonamide sensitivity tests, the first using a synthetic medium free from antagonizer (Kokko, 1947) and the second aiming at their removal from ordinary nutrient media (Harper and Cawston, 1945). Elaborate synthetic media have many disadvantages, and organisms may appear more sensitive than in the body. The removal of antagonizer from ordinary routine media by lysed horse cells, as described by Harper and Cawston (1945) and used by Evans (1948), necessitates careful estimation of the amount required for each batch of peptone; in most cases a high concentration of blood is needed and in this way false results may be obtained.