To determine whether propressophysin (vasopressin-neurophysin precursor) is present in human plasma, the nature of the immunoreactive neurophysin was characterized by gel filtration. When plasma samples obtained from six patients with the syndrome of inappropriate antidiuretic hormone secretion due to central nervous system disease were fractionated on a column of Sephadex G-50 in 0.2 N acetic acid, virtually all of the nicotine-stimulated neurophysin (NSN) immunoreactivity coeluted with 125I-labeled NSN. In contrast, gel filtration of plasma from six patients with oat cell carcinoma of the lung with ectopic vasopressin production consistently demonstrated, in addition, a peak of a higher molecular weight (HMW) form of neurophysin. This HMW neurophysin represented 8.7-29.4% of the total NSN immunoreactivity in plasma and its elution profile was not changed when chromatographed after incubation in 6 M urea. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the HMW neurophysin ran in the 20,000-dalton area of the gel. A substantial portion of the HMW neurophysin appeared to be a glycoprotein judging from its binding to Concanavalin A. When the HMW neurophysin was incubated with trypsin, most of the immunoreactivity was converted into a smaller neurophysin which bound to a vasopressin-agarose column in a pH-dependent manner. Moreover, a definite peak of immunoreactive vasopressin appeared after the trypsin treatment. This peak coeluted with synthetic arginine vasopressin on gel filtration and had the characteristic affinity of vasopressin for neurophysin-agarose. These results indicate that propressophysin circulates in patients with oat cell carcinoma of the lung with ectopic vasopressin production and suggest that plasma propressophysin may be a marker for ectopic vasopressin production.