Serum was obtained from two groups of 23 healthy female volunteers to determine the standard values of esterified fatty acids (EFA) contained in phospholipids, triglycerides and cholesterol. One group consumed an 'optimal' diet in regard to lipid and caloric content, under strictly controlled conditions, the other a free diet. All lipid fractions in serum from both groups were in the physiologic range so that the levels of EFA could be expected to be in the normal range. These data are comparable to those obtained from other series but they differ strongly from values established among the Eskimos, who have very low serum levels of arachidonic acid due to their traditional diet and probably to genetic factors. Determination of the standard values of EFA in healthy caucasian women may contribute to a better assessment of role of the therapeutic intervention on lipid parameters considered as atherogenic risk factors.