A noncompetitive enzyme-linked immunosorbent assay (ELISA) has been developed for measuring total plasma apolipoprotein (apo) B using affinity purified polyclonal and monoclonal antibodies. Microtiter plates from different manufacturers were tested with regard to their IgG binding characteristics; only one plate yielded consistent coefficients of variation of less than 5%. The optimal plasma dilution in this assay was 1:3000. IgG anti-apoB antisera conjugated to alkaline phosphatase was used as a second antibody. p-Nitrophenyl phosphate was utilized as substrate for color development, and the absorbance (410 nm) was read utilizing an ELISA reader interfaced with a microcomputer for data processing. Plasma apoB levels in plasma have been determined in 1115 male and female participants in the Framingham Offspring Study. Mean (+/- SD) plasma concentrations were 89 +/- 28 mg/dl. Significant age and sex related differences in apoB levels were noted.