The effect of egg yolk consumption on the composition of LDL and on the concentration of HDL subclasses was studied in healthy subjects. Six volunteers consumed a diet low in cholesterol for 10 days and then daily added 6 egg yolks to their diet for another 10 days; the experiment was repeated 1 year later with the same subjects. Egg yolk consumption caused the cholesterol intake to increase by 1600 mg/day, and the fat intake by 7 energy % at the expense of carbohydrates; this increase was due almost exclusively to monounsaturated fatty acids. Upon egg yolk feeding the mean level of serum total cholesterol rose by 13%; the bulk of this rise was due to LDL cholesterol, which increased by 21%. VLDL and IDL cholesterol decreased by 19 and 11%, and serum total triglycerides by 17%. Marked relative increases of 35 and 36% were seen in the cholesterol level of the HDL subfractions with densities of 1.055-1.075 g/ml (HDL1) and 1.075-1.100 g/ml (HDL2), respectively. The HDL2/LDL cholesterol ratio increased by 16%. No change in cholesterol in HDL3 (d greater than 1.100 g/ml) was observed. The increase in cholesterol in HDL isolated by density gradient ultracentrifugation significantly exceeded the increase in cholesterol in heparin-Mn2+ soluble HDL. This suggests the formation of apo E-containing HDL, i.e. HDLc, which has HDL density but is not soluble in heparin-Mn2+. The composition of the LDL particles was significantly altered; the core became enriched in esterified cholesterol at the expense of triglycerides, and the ratio of core components to surface components increased by 7%.