The question of a possible synergistic or antagonistic effect during treatment with combinations of antibiotics is of great importance. I n particular this holds true in the treatment of infections caused by more than one microorganism or by different strains of the same bacteria. Such Combination treatment is often indicated also in the treatment of infections from only one single microorganism, inasmuch as a certain tendency to increased resistance is found after the use of one antibiotic only for a longer time.The main sphere of action of penicillin being gram-positive and gram-negative cocci and that of streptomycin being grani-negative and acidfast bacilli, it is particularly combinations of penicillin and streptomycin that have come to the fore.In the U. S. A. combinations of different antibiotics have been studied mainly by Jawetz et al., who were the first to prove that the combination penicillin plus streptomycin showed a synergistic effect. This was done by in vitro tests against certain strains of Enterococci and B. proteus. On Streptococci-inoculated mice a synergistic effect, could be shown also in vivo.Jawetz & Gunnison (1953) have summarized their own points of view and also rcsults from other studies published in this field. They conclude that the antibiotics generally in clinical use can be divided into two groups: I. Penicillin, streptomycin, bacitracin, and neomycin. 11. Chlortetracycline, chloramphenicol, and oxytetracycline.They also state that ))Members of group I are frequently synergistic with each other, occasionally indifferent, never antagonistic. Members of group I1 are neither synergistic with nor antagonistic to each other, but simple additive effects are often observed, which presumably also could be obtained by an increase in the dose of a single drug. When a member of group I is added to one of group 11, the combined effect is a function of the microorganisms' behavior, and is not predictable a pri0ri.o