Radial immunodiffusion (RID) has been used for determination of low density lipoprotein (LDL) B protein in plasma. During measurement of B protein in plasma and the d less than and d greater than 1.019 g/ml plasma fractions by RID in 1.0%, 1.5%, 2.0%, and 2.5% agarose, the d less than 1.019 g/ml lipoproteins diffuse in the agarose and produce precipitin rings. Among normotriglyceridemic subjects, the B protein values in whole plasma obtained by RID using 1.5 to 2.5% agarose were only slightly higher than the values in the d greater than 1.019 g/ml fraction obtained by RID and closely approximated the values obtained in the d greater than 1.019 g/ml fraction by radioimmunoassay. However, among the hypertriglyceridemic subjects, the RID measurement of B protein in plasma using 1.0 to 2.5% agarose overestimated the LDL B protein levels in plasma. The RID procedure at agarose concentrations of 1.5% to 2.5% can be used to estimate plasma LDL B protein levels in normotriglyceridemic subjects. However, measurement of LDL B protein by RID in plasma of hypertriglyceridemic subjects must be interpreted with caution; the LDL B protein is overestimated by this procedure because of the contribution by the d less than 1.019 g/ml lipoproteins to the B protein value.