Apoproteins A-I, A-II, C-II and E were determined in 94 patients with various hyperlipidemias and 87 normal controls using single radial immunodiffusion (SRID) method recently developed by Daiich-Kagaku Co. Apo A-I and A-II correlated positively with HDL-cholesterol and correlation coefficient values (r) were 0.620 and 0.465 respctively. There was a strong correlationship between apo C-II and serum triglyceride (r=0.748). Apo E correlated with both of serum cholesterol and triglyceride and r values were 0.555 and 0.511 respectively. Apo C-II could not be detected in a case with type I hyperlipoproteinemia caused by Apo C-II deficiency, confirming that the anti-apo C-II serum used in the SRID plates was immunochemically pure. The ratio of Apo C-II over triglyceride correlated positively with HDL-cholesterol, suggesting this ratio might express the state of lipoprotein metabolism. Although apo E levels in type III hyperlipoproteinemia with apo E3 deficiency was significantly higher than in normal and the other hyperlipidemic subjects in agreement with the previous papers, there were substantial overlaps of apo E levels between type III and the other hyperlipidemia such as type V. Howerer, when the ratio of apo E over total cholesteriol was calculated, type III hyperlipoproteinemia with apo E3 deficiency could be clearly discriminated from the other hyperlipidemia and normal controls. Therefore, the ratio of apo E/total cholesterol, can be easily available for the diagnosis of type III hyperlipoproteinemia with apo E3 definceency.