In 1953 Gale and Folkes (1) observed that chloramphenicol inhibited protein synthesis in Staphylococcus aureus but permitted substantial nucleic acid synthesis to continue. Subsequently, this drug has served as a powerful tool in studies of protein and nucleic acid metabolism in a wide variety of microbial systems. In sensitive bacterial cultures the addition of 30 to 100/~g/ml 0.1-0.31m~) of chloramphenicol results in prompt and nearly complete suppression of protein synthesis (1, 2), while concentrations as low as 2 ~g/ml (0.006 m~) produce 50 per cent inhibition in some bacteria (1). Very low oncentrations also inhibit protein synthesis in cell-free bacterial systems (3-7). On the other hand, in mammalian cells these levels of drug have only infrequently been reported to have demonstrable metabolic effects (see Discussion). The pathway for protein synthesis appears to be quite similar in microbial and mammalian cells, but the discrepancy in their sensitivity to chloramphenicol remains a puzzle.

Antibody production in vitro became a practical system for studying protein synthesis by mammalian ceils following the improvement of tissue culture methods and the development of the hemagglutination assay for antibody. This paper deals with the effect of low levels of chloramphenicol (5 to 50 ~g/ml or 0.015-0.15 IBm) on antibody production in cultures of stimulated rabbit lymph node fragments. Such "bacteriostatic" levels of the drug have been found to cause marked suppression of the secondary response induced in vitro.