Immunoturbidimetric assays for determination of serum apolipoproteins CII and CIII (apo CII and CIII) were developed for use on the Cobas-Bio centrifugal analyzer with commercially available antisera and reference sera. Within-run and between-run imprecision (CVs) for apo CII was respectively 3.3 and 6.2%, and for apo CIII 2.75 and 5.2%. Addition to sera of as much as 50 g of hemoglobin or 0.15 g of bilirubin per liter did not interfere with determinations of apolipoproteins. Concentrations of apo CII and CIII were measured in sera from 40 normolipidemic and hyperlipidemic subjects by these immunoturbidimetric methods (x) and by radial immunodiffusion assays (y). Apo CII and CIII gave slopes of 0.93 and 0.86, y-intercepts of 7.4 and 9.5 mg/L, and correlation coefficients of 0.96 and 0.94, respectively. Reference intervals for the two apolipoproteins were established with sera from 100 normolipidemic subjects. The distribution of apo CII and CIII among the various lipoprotein fractions for normal subjects differed significantly (p less than 0.005) from that for hypertriglyceridemic subjects.