Abstract
In patients with vitamin B12 deficiency, phytohemagglutinin-stimulated cultured lymphocytes had little or no thymidylate synthetase activity. Ample activity was found in such lymphocytes from normal individuals and patients with folic acid deficiency or pernicious anemia in remission. It therefore appears that the megaloblastosis that is associated with vitamin B12 deficiency is related to low thymidylate synthetase activity.