Effects of endotoxin on the immune response to sheep red cells added in vitro to spleen cell suspensions from unimmunized mice were studied. The optimal dose of endotoxin (1 μg/ml) gave a six-fold increase in the number of plaque-forming cells per 10(6) viable cells, after 6 days of culture, if it was present from the 12th to the 24th hour after antigen. There was little or no enhancement when endotoxin was given at other times during the 6 days of culture. The response was depressed when the suspension was incubated with endotoxin before the addition of red cells. The periods of RNA and DNA synthesis were determined by adding tritiated uridine and tritiated thymidine, respectively, for 24-hour periods during the 6 days of culture. Synthesis of RNA occurred throughout the culture period whereas DNA synthesis was not detected before 24 hours of culture. We found that the adjuvant action of the endotoxin which was maximum during the 1st day of culture, was associated with an initial phase of RNA synthesis, before DNA synthesis could be detected
