Lymphocyte blastogenesis by stimulation of PHA was compared in 17 normal subjects and 11 patients with cancer. The blastogenesis was measured by incorporation of ³H-thymidineinto lymphocytes, using microplate assay. It was disclosed that the blastogenesis was significantly decreased in patients with cancer than normal subjects. When indomethacin was added to this system at the concentration of 1μg/ml, the lymphocyte blastogenesis in patients with cancer was increased by 42.0%, while the blastogenesis in normal subjects was increased by 17.6%. These results might suggest that prostaglandin secretion by adherent cell is at least partly responsible for the depression of the blastogenesis in patients with cancer.