Streptomycin is effective against a variety of gram negative and gram positive bacteria both in vivo and in vitro.1 However, its rational use in the treatment of disease must be preceded by an evaluation of dose intervals and sizes with a determination of bacterial susceptibility to its action. There are definite in vitro differences in the sensitivity of bacteria to streptomycin and within a given species strains may be found varying from extreme susceptibility to high resistance. As the blood concentration of streptomycin reaches a peak shortly after parenteral injection and then gradually falls to zero2 it becomes necessary, for rational therapy, to determine whether a sufficiently high blood concentration is being maintained with reference to the sensitivity of the bacteria isolated from the patient. In many cases it may be found that a smaller dose or a longer interval between doses is sufficient to maintain a blood