We report our experience with a method to evaluate binding and uptake in cells of low density lipoprotein (LDL) from heterozygous patients with familial defective apolipoprotein B-100 (FDB-LDL) and LDL from normolipidemic subjects (nonFDB-LDL). The method is based on competition for binding/uptake in Epstein-Barr Virus (EBV)-transformed lymphocytes or COS cells overexpressing an LDL-receptor transgene between fluorescently labeled LDL and the unlabeled LDL of interest, and measurements are by flow cytometry. With EBV-lymphoblasts, the ability of FDB-LDL to displace fluorescent LDL ("Dil"-LDL) from cells at 4 degrees C (binding) was reduced to approximately 1/3 of normal. Displacement of "Dil"-LDL by FDB-LDL from cells at 20 degrees C (binding/uptake) was reduced to less than 1/2 of normal. Similar results were obtained with COS cells. Freezing of serum to -80 degrees C for 24 hours did not affect results, and we could discriminate between binding/uptake of FDB-LDL and nonFDB-LDL prepared from serum that had been stored at -80 degrees C for three months.