Abstract
1. The effect of caffeine at concentrations of 10(-2)M to 10(-4)M on the mitotic rate of phytohaemagglutinin-stimulated human lymphocytes from twelve normal subjects was examined in vitro.2. At 10(-2)M the drug was cytotoxic.3. At 10(-3)M caffeine reduced the rate of mitosis in all cases.4. At 10(-4)M the drug was found to reduce the rate of mitosis in some cases, to give the same value as the control cultures in others and in a third group to increase the mitotic rate.5. At 10(-4)M all the lymphocytes from females showed an increased rate of mitosis and all but one of the lymphocyte samples from males showed a decrease or no change. Thus at this concentration the effect of the drug on the mitotic rate appeared to be related to the sex of the lymphocyte donor. There was also a significant difference in the degree of inhibition between male and female lymphocytes at 10(-3)M.