Chloramphenicol is readily absorbed from the gastrointestinal tract. Peak serum levels, while usually reached within two hours, are occasionally not attained for six or more hours. In general absorption is markedly more rapid than in the case of aureomycin, which seems to be absorbed slowly over a period of several hours. Eleven mg./kg. body weight, which is an adequate single dose of aureomycin, in the case of chloramphenicol produces serum levels too low to be detected by the authors' method. Twenty-two mg./kg. produces two hour levels averaging 8.3 µg./cc. and 44 mg./kg. raises this level to a mean of 22 µg./cc. Rectal administration of perforated chloramphenicol capsules produces peak serum levels approximately one-fifth of those effected with comparable oral dosage. Diffusion of chloramphenicol into cerebrospinal fluid occurs readily and after a few days administration spinal fluid levels, in three cases, were approximately 50% of the serum levels. These data suggest that effective oral dosage of chloramphenicol ranges from 22 to 44 mg./kg./dose and that the antibiotic should be given at 6 to 8 hour intervals.