The separation and characterization of 25-hydroxyvitamin D3 3-sulfate in human plasma are carried out by high-performance liquid chromatography. The vitamin D sulfate fraction is obtained from a plasma specimen (three volunteers) by the combined use of a Sep-Pak C18 cartridge, for solid-phase extraction, and a lipophilic gel (piperidinohydroxypropyl Sephadex LH-20), for ion-exchange chromatography. 25-Hydroxyvitamin D3 3-sulfate is identified in the examined three specimens in two ways: its chromatographic behavior and that of its fluorescent labeled derivative using 4-[4-(6-methoxy-2-benzoxazolyl)phenyl]-1,2,4-triazoline-3,5- dione; and data obtained from the solvolysis reaction.