When lymph node fragments from previously immunized rabbits were stimulated in vitro to produce a secondary response, the continuous presence of 50 µg/ml (0.15 mM) of chloramphenicol in the medium during the entire incubation period of 15 to 21 days produced nearly complete suppression of the response. Concentrations as low as 5 µg/ml (0.015 mM) produced approximately 80 per cent suppression of the response.
When 50 µg/ml of chloramphenicol was present during only the first 6 days of culture, the secondary response was reduced 90 per cent. When it was absent for the first 6 days but present for the next 9 to 15 days, the response was reduced only 40 per cent. Since over 95 per cent of the antibody of the secondary response in most experiments appeared in the medium after the 6th day, chloramphenicol apparently inhibits antibody production by interfering with some early phase of the response. It is suggested that this interference involves messenger RNA and that animal cells have appeared resistant to this drug only because their complement of messenger RNA present when the drug has been added is stable over the short periods during which protein synthesis has usually been studied.