The relationship between the content of probucol in plasma and lipid lowering was studied after administration of probucol. The study group consisted of 44 patients with Type II hyperlipidaemia (mean age 56 y). Total cholesterol level was decreased from 293 to 232 mg.dl-1 by the administration of probucol. The plasma probucol concentration was 2.03 mg.dl-1 after 4 weeks of administration of 500 mg b.d. Although there was no relationship between the change in total cholesterol and the plasma probucol, a linear relation was observed between the decrease in LDL-cholesterol concentration and the probucol concentration in LDL. Neither the VLDL nor the HDL-probucol content were related to the magnitude of the decrease in lipoprotein cholesterol. The present study has demonstrated that an increase in the probucol content in LDL was associated with a decrease in plasma LDL-cholesterol.