Low density lipoproteins (LDL) containing apolipoprotein B were separated from 15 fresh normal human serum pools by three independent isolation methods including sequential ultracentrifugal flotation, affinity chromatography, and polyanion precipitation. A discrete subpopulation of LDL (LDL-In) was isolated which possessed comparable inhibitory activity for PHA, PWM, and allogenic cell stimulated human lymphocytes in vitro at concentrations of 1 to 10 mug protein/1 x 10(5) lymphocytes/0.25 ml culture. LDL-In was characterized by a mean buoyant density of 1.055 g/ml in KBr, a m.w. of 2 to 3 x 10(6) daltons and a composition of 20 to 25% protein and 75 to 80% lipid with beta electrophoretic mobility. The biologic activity of LDL-In was non-cytotoxic, independent of mitogen concentration, and dependent upon the concentration of serum in the culture assay. The effect was temporally dependent requiring approximately 24 hr for induction of a stable suppressed state. Suppression was reversible with shorter periods of exposure to LDL-In. LDL-In did not inhibit lymphocytes at periods greater than 19 hr after stimulation, suggesting that LDL-In may influence metabolis events associated with the inductive phase of lymphocyte activation by lectins and allogeneic cells. LDL-In was clearly distinguishable from T lymphocyte E rosette inhibitory factor since it did not influence E rosette function of lymphocytes. The physicochemical and biologic properties of LDL-In clearly distinguish this reguloratory lipoprotein from previously described immunoregulatory factors.