By using the apparatus demonstrated in the first report, the author has studied the lipoprotein of serum, extending at 110 v. for 7 hours filling the space with air and staining with Sudan black B (S. B. B). The following results have been obtained. 1. From the integrated values of the area obtained by densitometry on the papers stained with S. B. B it is possible to know roughly the amount of lipoprotein of serum and exact quantitative relation between α and β lipoproteins. 2. The drying of paper causes the masking of the lipids of lipoprotein which can be unmasked again by heating at 110℃. for 20 minutes. 3. Pepsin, phenol and some carboxylic acids act as to unmask the masked lipoprotein. After unmasking some lipids escape into the solvent during the S. B. B staining in α-lipoprotein, but not in β-lipoprotein. 4. From the observations on the masking and unmasking of serum lipoproteins the binding modus between lipid and protein and the masking-unmasking mechanisms have been discussed.