ApoC-II and apoC-III of human very low density lipoproteins (VLDL) have been quantified by analytical isoelectric focusing (IEF) between pH 4 and 6 in polyacrylamide gels containing 8 M urea. The isoelectric point of apoC-III0 is pH 4.93; apoC-II, pH 4.78; apoC-III1, pH 4.72, and apoC-III2, pH 4.54. ApoC-I is not found in the pH range between pH 4 and 6. Two minor peptides, apoC-IV and apoC-V, with isoelectric points of pH 4.61 and 4.44, respectively, are apoproteins not previously identified. The sensitivity (5--40 microgram) and reproducibility (+/- 8%) of this method allow quantitative analysis of apoC-II and apoC-III distribution in VLDL.