A method for separating high density lipoproteins from blood serum or plasma. The method involves contacting the blood sample with an absorbant material comprising porous silica or silicate to adsorb the high density lipoprotein in preference to other lipoproteins in the blood sample. When combined with means for removing very low density lipoproteins and chylomicrons from the blood sample, the remaining low density lipoproteins can be measured directly.