Ascidians specifically accumulate high levels of vanadium from seawater in their blood cells. Almost all of the vanadium is present in a reduced form in the blood cells, although the metal exists in a +5 oxidation state in seawater. It has, therefore, been assumed that agents that cause the reduction of vanadate(V) to vanadyl(IV) must be present within ascidian blood cells. In this regard, we have extracted a vanadium-binding substance, which we have called vanadobin, from the vanadocytes of ascidians. We examined whether vanadobin is involved in the reduction of vanadate(V) accumulated from seawater. Data obtained by spectrophotometry and ESR spectrometry revealed that not only a crude homogenate of vanadium-rich blood cells but also a purer form of vanadobin eluted from a column of Sephadex G-15 could reduce vanadate(V). Our experiments demonstrate that vanadobin, a vanadium-binding substance extracted from ascidian blood cells, can reduce vanadate(V) to vanadyl(IV) and maintain it in the reduced form.