Analysis of the plasma from a totally pancreatectomized patient, with antiserum 30 K, has demonstrated basal glucagon immunoreactivity (GIR) levels in the normal range (80-110 pg/ml). Neither i. v. arginine nor oral glucose affected these GIR values, thus indicating the absence of functioning pancreatic or gastrointestinal A-cells. Furthermore, filtration of whole plasma on Bio Gel P-30 showed no GIR in the 3500 MW elution volume. GIR was found to be distributed in two peaks. One peak eluted in the protein region, similarly to "big plasma glucagon" (BPG), and the second peak appeared after the glucagon-I125 marker. The protein-sized moiety was not absorbable by charcoal, and on Sephadex G-100 it eluted within the globulin region. When subjected to trypsin treatment, it yielded smaller GIR fractions. According to these criteria, it can be assumed that this component is identical to BPG. Therefore, an extrapancreatic source for BPG is suggested. On the other hand, the presence of fasting hyperglycaemia in this patient indicates that insulin deficiency by itself suffices to raise blood sugar to diabetic levels.