Objectives: To study the mechanisms of activation of T lymphocytes, the nature and products by which they activate macrophages and their role in cytolysis of tumors. Approach: Because products of activated T-lymphocytes are biologically active in minute quantities, we have been studying their biochemical nature by isotopic labelling techniques, both double and single amino acid labels followed by fractionation by gel electrophoresis and radioautography. Because both B and T lymphocytes make MIF and all other known lymphokines, proteins produced only by T lymphocytes are being sought, particularly in human lymphocyte populations.