Immunological methods permit separation and measurement of lipoprotein concentrations in terms of their apolipoprotein composition. We have elaborated two different methods to measure lipoprotein particles in plasma. The first is a selective bi-site Elisa procedure and the second a differential electroimmunoassay. Lp A-I plays an essential role in reverse cholesterol transport. We have shown that Lp A-I is an anti-risk marker of atherosclerosis. Its concentration is higher in females than in males, it is decreased in patients with coronary heart disease and in their kindreds and in populations with a high cardiovascular risk. This fraction is increased in octogenarians. In pharmacological terms, HMG CoA reductase inhibitors increase Lp A-I and Lp A-I:A-II while fenofibrate decreases Lp A-I and increases Lp A-I:A-II. These studies demonstrate that the measurement of lipoprotein concentrations according to their apolipoprotein composition can reveal lipoprotein fractions that have precise metabolic roles with epidemiological and pathophysiological implications.