High-density lipoproteins (HDL) turn turbid during in vitro incubation, concomitant with the formation of a cholesterol ester-rich lipoprotein, designated HDL-sup. The increase in turbidity (A) formed in relation to incubation time (t) is an asymptotic function: A=Uo(1 - e-k1t), where Uo is the amount of HDL with the property of releasing HDL-sup and k1 the velocity constant of the reaction. The increase in turbidity and formation of HDL-sup was not related to cholesterol ester content of the incubated fraction nor to exogenous factors like bacterial growth. The in vitro incubation was accompanied by a cholesterol esterification with a mean production of 8 nmol cholesterol ester/mg HDL protein, but also by a more pronounced degradation of phosphatidyl choline, 148 nmol/mg HDL protein. These data indicate that the lipid changes are induced by a two-step lecithin:cholesterol acyltransfer (LCAT) reaction. This reaction caused in HDL lipids a consumption of surface material and an increase in 'lipid core', presumably leading to a weakening and disruption of the lipoprotein surface with a recombination of 'lipid core' material in the form of HDL-sup.