Separate electroimmunoassays are described for measuring human plasma apolipoproteins C-I and C-II. Purified apolipoproteins C-I and CII were used in preparing monospecific antisera and as the primary standards. These assays are sensitive (maximal sensitivity, 20 ng), specific, rapid, precise (the within- and between-assay coefficients of variation for both assays were 5 and 8%, respectively), and accurate (accuracy was based on comparison of calculated and measured C-I, C-II, and C-III contents of an ApoC-containing column-eluent fraction) and are applicable to measurement of C-I and C-II polypeptides in whole plasma and density classes. However, plasma samples with triglyceride (triacylglycerol) concentrations greater than 6000 mg/L must be delipidized before analysis for C-II, as must those with greater than 12 000 mg/L before analysis for C-I polypeptide. Mean concentrations (and SD) of C-I in plasma of normolipidemic subjects and hyperlipoproteinemic phenotypes IIa, IIb, IV, and V were 60 (15), 70 (20), 100 (20), 100 (20), and 260 (94) mg/L, respectively. The corresponding C-II values were 40 (20), 43 (20), 68 (20), 65 (20), and 210 (70), respectively. C-I and C-II concentrations in patients with phenotypes IIb v, IV, or V significantly (p less than 0.001) exceeded those in normal persons or phenotype IIa. The observed correlations (r = 0.92 and r = 0.94) between triglyceride and C-I and C-II values suggest that these two polypeptides, like C-III, are excellent plasma markers for assessing the state of triglyceride metabolism.