(1) The treatment of 23 cases of acute lobar pneumonia with M and B 693 is described. There were 3 deaths, none of which can be ascribed to a failure of drug action. No controls were available, but this result compares very favourably with the usual mortality of the disease. (2) The drug had a striking effect on the temperature, which in most cases was reduced to normal within 24 hours. Clinical improvement was.rather slower, occurring usually in 48 hours. (3) There appeared to be no effect on the rate of resolution of the signs in the chest, which disappeared at about the same time as would have been expected in cases treated without the drug. There were no cases of delayed resolution. (4) The drug did not seem to be effective in preventing the onset of complications, as 3 cases developed an empyema. However, the small number of patients involved does not exclude the effect of coincidence. No sterile effusions were encountered. (5) A case of so- called Friedlander pneumonia is, described. The drug seemed to have a considerable action against the pneumobacillus, although it did not prevent the development of an ampyema. The patient survived. (6) Four cases of "staphylococcal pneumonia" were encountered. One was moribund on admission and rapidly died, but the others showed a good response to the drug. M and B 693 therefore appeared to possess a powerful anti-staphylococcal action. (7) One case had a mixed growth of haemolytic streptococcus and Staphylococcus aureus in his sputum. There was a prompt response to the drug. This helped to confirm the anti- staphylococcal action of M and B 693, and also suggested that it Was active against the haemolytic streptococcus. (8) The only serious toxic effect encountered was the fairly frequent occurrence of vomiting, which in some cases was a serious impediment to -the use of the drug. í`o way of overcoming this difficulty was discovered, short of withdrawing the drug. The drug did not seem to have any toxic action on the white blood cells. (9) The final conclusion reached was that :I and B 693 is an extremely effective agent in the treaLment of acute pneumococcal or staphylococcal lobar pneumonia, and that it may be of considerable value in the treatment of Friedlander pneumonia.