The discovery of the effect of prontosil by Domagk in 1935 marked a new era in the treatment of bacterial disease. Later in the same year Tréfouel and his colleagues found that sulphanilamide, a simpler substance than the original prontosil was just as effective, and during the last 4 years a large number of compounds of sulphanilamide have been made. Compounds in which substituents are introduced into the amino group of sulphanilamide include the original prontosil (now called prontosil rubrum), prontosil soluble, rubiazol, proseptasine and soluseptasine; they probably owe their activity to the liberation of sulphanilamide in the body and they are not more active than sulphanilamide itself. The soluble derivatives for parenteral injection are only indicated when treatment by mouth is impossible, and if treatment by this route has to be used exclusively, large doses are required, e.g., 60 to 120 c.c. daily. Compounds in which substituents are introduced into the amide group of sulphanilamide, on the other hand, are probably not broken down in the body, and one of these—M. & B. 693—has a wider range of activity than sulphanilamide itself. It is much more effective in pneumococcal conditions and slightly more effective in staphylococcal and gonococcal conditions. Uleron and albucid, two other compounds of this class are used in gonococcal conditions. Many tropical infections have been found to be benefited by these drugs. Elephantiasis of filarial origin is rapidly cleared up owing to the action of the drugs on the secondary streptococcal infection. In malaria, P. knowlesi infections of monkeys are rapidly cured, but P. relictum in birds and P. falciparum, P. vivax and P. malariae infections in man are very much less susceptible. The drugs, however, appear to act as a true causal prophylactic in P. falciparum infection in man, and if patients are treated at the time of the mosquito bites, infections do not occur. These are the only known drugs to produce this effect. Cases of pneumonia are improved in a remarkable way by M. & B. 693 and meningococcal meningitis responds well to both sulphanilamide and M. & B. 693. In typhoid fever some effect may be expected in the first week of the disease, but the benefit of drug treatment in the later stages is much more doubtful. In Brucella abortus and B. melitensis infection good effects are reported, but they are perhaps not so striking as those in streptococcal, meningococcal and pneumococcal conditions. In plague, experimental results with M. & B. 693 have been remarkable, but there is very little published clinical work. The only virus conditions which are directly influenced are lymphogranuloma and trachoma; clinical reports on these conditions indicate that in the former the treatment is effective, while in the latter the corneal condition is improved much more than that of the conjuctiva. In smallpox, exhibition of the drugs prevents the pustulation which is responsible for most of the trouble in this disease. Administration of the drugs orally or subcutaneously enables the blood to kill large numbers of streptococci. If sulphanilamide is added to shed blood, streptococci grow at the same rate in the sulphanilamide specimens as in the controls for the first 2 hours, but subsequently the streptococcal population in the sulphanilamide specimens decreases until the blood becomes sterile, while in the controls the organisms go on multiplying. The mode of action of the drugs on the bacteria is still undecided.