The recently proposed calibrant LHN-1 (lot F537; henceforth designated F537), for the molecular weight (MW) determination by high-performance size-exclusion chromatography of heparins, is shown here to have a range too narrow to allow for the accurate MW determination of all low molecular weight heparins (LMWHs). We have recently demonstrated, by this same methodology, that a chemically degraded benzyl ester of unfractionated heparin, heparin mass calibrator (HMC), is a better calibrant. Weight-average MW, number-average MW, peak MW, and dispersity values were calculated with F537, HMC, and by a reference narrow-range-calibration method for various LMWHs and unfractionated heparins. Values for these parameters determined with HMC were not significantly different from those determined by the reference method until the MW of the substance exceeded 15.0 kDa. In contrast, the MW profile obtained with F537 was appreciably different from that obtained by the reference method for samples with MWs > 7.5 kDa. The range exhibited by HMC should allow this calibrant to be used for both LMWHs and unfractionated heparins.