A solid phase radioimmunoassay (RIA) has been developed for apolipoprotein B (apoB), a major constituent of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) in man. Antisera were prepared against apoB in goats and rabbits, coupled to bromoacetyle cellulose, and the complex was incubated with [125I] LDL. The RIA was based on the displacement of [ 1252]-LDL by unknown samples, as compared with unlabeled LDL standards, using a logit transformation to calculate results. The RIA was found to be satisfactory in terms of precision, sensitivity, reproducibility and specificity. Control subjects had mean apoB levels of 94 mg/100 ml in whole fasting plasma, of which 3.6 mg/100 ml plasma were in the VLDL, while 86 mg/100 ml plasma were in the LDL. Both the triglyceride and apoB content of VLDL, and the cholesterol and apoB content of LDL were positively correlated. It is concluded that the solid phase radioimmunoassay described in the present report provides a rapid and relatively simple means of quantitating apoB in normal human plasma.