The blastogenic and DNA synthetic response of human peripheral blood lymphocytes (PBL) to phytohemagglutinin (PHA) and allogeneic cells can be inhibited by a nontoxic aqueous extract (LEx) of normal human liver. LEx reversibly inhibits the activation of PBL by PHA, arrests ongoing DNA synthesis, and limits the duration of the DNA synthetic response to PHA at concentrations as low as 0.7 to 1.5 µg LEx protein/culture. In contrast, human T lymphocyte E rosette formation is unaffected by LEx concentrations in excess of 900 µg/culture. LEx has been partially purified by ultracentrifugation, ammonium sulfate precipitation, and molecular exclusion chromatography and appears to be a heat labile protein with a m.w. of approximately 65,000 and an isoelectric point of approximately 4.08. LEx is distinct from other previously described human immunoregulatory molecules and is potentially releasable in vivo from injured or necrotic liver cells. Because of its potency and anatomic distribution LEx may potentially modulate immunopathogenetic events responsible for assorted inflammatory and neoplastic liver diseases.