Lymphocyte proliferation was determined by uptake of 3H-thymidine at 18 hours before harvesting. The radioactivity was measured by a liquid scintillation counter. The degree of activation was expressed as a stimulation index [S.I.,i.e., the ratio of the 3H-thymidine uptake in countper minute (CPM) of samples with extract to those without extract]. Phytohemagglutinin at 10 ìg/ml was also added to the culture system to check for cell survival and used as a positive control of each assay.