Familial hypercholesterolemia(FH) is a disease based on defects of low-density lipoprotein receptors(LDL-R). To interrupt and control the natural course of this disease, early identification of these patients is important. The routine lipid profile tests for hypercholesterolemia can not differentiate objectively FH from secondary hypercholesterolemia. The exact diagnosis of FH heterozygotes is especially essential because it is easier to develop premature coronary heart diseases compared with secondary hyper-cholesterolemia. A simplified rapid and precise method for the mass screening of FH patients and the differentiation between FH heterozygote and secondary hyperlipidemia was needed. For the test, lymphocytes were used as target cells in LDL-R assay. After a 5 day culture with anti-CD3 Ab as a mitogen, indirect immunofluorescence stain and flow cytometric analysis were applied. The results were as follows; 74 ± 9% of the stimulated lymphoblasts from normal controls expressed LDL-R activity. Cultured, but unstimulated, lymphocytes of normal controls showed 27 ± 8% positivity and total cultured lymphocytes showed positivity of 46 ± 11% positivity. Lymphoblasts, unstimulated lymphocytes, and total cultured lymphocytes from hyper-cholesterolemia without FH showed 74 ± 10%, 25 ± 10% and 50 ± 17%, respectively, which showed no significant differences from normal control groups. FH Heterozygotes showed LDL-R positivity, 21 ± 11% in lymphoblasts, 11 ± 6% in unstimulated lymphocytes and 18 ± 7% in total cultured lymphocytes. These data imply that adequately stimulated lymphocytes might be used for detecting defects in LDL-R and used to differentiate FH from secondary hypercholesterolemia.