At the present time the most useful technique with which to examine the kinetics of lowdensity lipoprotein (LDL) cholesterol in vivo is the labeled LDL turnover study. However, a major limitation of this method is that, despite its ability to accurately measure both the plasma LDL concentration and LDL production rate, it cannot directly quantify LDL receptor activity. The present study defines the equations that describe the relationship between LDL cholesterol production rate, LDL receptor number, and plasma LDL cholesterol concentration. These equations provide a method that allows calculation of total LDL receptor activity based on the results of an LDL turnover study. With the use of this technique and data from previously published series, the effects of the genetic absence of receptors, aging, and the treatment of hypercholesterolemia with mevinolin on LDL cholesterol kinetics were analyzed.