Receptor-dependent uptake of low density lipoproteins (LDLs) labeled with the fluorescent probe 3,3'-dioctadecylindocarbocyanine iodide (DiI) by human lymphocytes was measured by flow cytometry. Freshly isolated lymphocytes showed very little uptake, whereas the cells cultured in lipoprotein-free medium for 48 hours showed about a 10-fold increase in the uptake of DiI-LDL. The uptake was inhibited by excess unlabeled LDL, heparin, and ethyleneglycol-bis-(beta-aminoethylether)-N,N, N',N'-tetraacetic acid (EGTA) but not by acetylated LDL, and therefore it appears to be very specific. Comparative studies with iodine 125-labeled LDL showed that there is correlation between the uptake of DiI-LDL and degradation of 125I-labeled LDL by the lymphocytes. The assay was carried out with lymphocytes from several normal healthy individuals and some clinically diagnosed heterozygous familial hypercholesterolemic patients. The results indicate that this assay can be used to distinguish patients with LDL receptor-defective familial hypercholesterolemia from normal individuals.