High-density lipoproteins (d=1.095--1.21) (H.D.L.( were isolated from six healthy men and women who added 4 to 6 eggs per day to their diet for 4 weeks and from five individuals who gradually increased their egg consumption to 3 per day over an 18-week period. Pre-diet and post-diet H.D.L.-binding activities for the cell-surface receptors of fibroblasts were compared by determining the quantity of 125I-labelled low-density lipoprotein which was competitively displaced by H.D.L. in binding, internalisation, and degradation assays. Irrespective of whether plasmacholesterol changed during the course of the diet, the binding activity of the post-diet H.D.L. was enhanced 2.6-fold to 4-fold compared with pre-diet activity. Furthermore, the increased binding activity in the H.D.L. could be accounted for by a minor, but potent, H.D.L. subfraction precipitated by heparin/manganese. Both binding activity and heparin precipitability appeared to correlate with an increase in arginine-rich apoprotein (apo-E) in the active H.D.L. subfraction. These data show that consumption of large numbers of eggs, whether or not it leads to an increase in plasma-cholesterol, does alter the properties of human H.D.L.