Radial immunodiffusion (RID) was used for determination of apolipoprotein B in LDL, VLDL, and plasma. The method closely approximated the values of LDL apo B obtained by the standard Lowry procedure (r=0.968). Using 1% agarose in the method made it feasible to estimate plasma and VLDL apo B levels. The present study also shows that plasma total cholesterol level correlates well (r=0.981) with the plasma apo B level measured by the RID method.