1. Measurements were made of the maximal low-density lipoprotein (LDL) receptor activities of blood lymphocytes from 81 healthy men with a wide range of plasma LDL cholesterol concentrations (1.45-7.55 mmol/l). 2. Receptor activity was quantified by measuring the degradation of 125 I-labelled LDL (10 μg of protein/ml) to trichloroacetic acid-soluble material during a 6 h incubation, after derepression of the lymphocytes for 72 h in lipoprotein-deficient medium. 3. No significant correlation existed between LDL receptor activity in vitro and plasma LDL cholesterol concentration in vivo ( r = −0.08).