Fluorescein-isothiocyanate (FITC) labeled anti-LDL-IgG was used to demonstrate LDL bound with lymphocytes. The activity of LDL receptor was expressed by the fluorescence intensity of individual cell (receptor-ligand immunofluorescence method). With this method the effect of dietary cholesterol on LDL receptor activity of lymphocytes was studied. The results showed that with the elevation of plasma cholesterol, the fluorescence intensity of individual cell and the percentage of labelled cell were decreased. i.e. the LDL receptor activity was reduced due to hypercholesterolemia, and lowering of plasma cholesterol level might be accompanied with restoration of the LDL receptor activity. These results suggest that inhibition of LDL receptor activity in patients with non-familial hypercholesterolemia can be recovered by decreasing plasma cholesterol level.