We have recently described a technique for measuring LDL size by high performance gel-filtration chromatography (HPGC) with UV detection (Scheffer et al., Clin Chem 1997;43:1904-12). A drawback of this method is the necessity of LDL isolation before chromatography. We now describe a modification of this method based on selective detection of lipoproteins by postcolumn labeling with parinaric acid, a fluorescent lipid probe. Measuring the size of isolated LDL by HPGC in 56 subjects, we obtained diameters of 25.72 +/- 0.60 nm with UV detection and of 25.74 +/- 0.58 nm with fluorescence detection. The modified method is suitable for LDL size measurement in whole plasma or serum. LDL sizes measured in whole plasma correlated strongly with the respective values in isolated LDL (r = 0.976) but were on average 0.18 nm larger (P < 0.001). CVs for within- and between-series imprecision were <0.25%. The present method requires only 5 microL plasma or serum without sample preparation and is suitable for the unattended analysis of large series of samples.