The isolated pancreatic glands from six pigs were perfused in a single pass system and then stimulated with arginine. Samples of the early response perfusate were subjected to gel filtration. Glucagon was determined radioimmunologically in the column effluents using four different antisera with various capacities to recognize other molecular forms of pancreatic type glucagon immunoreactivities. One antiserum crossreacted with purified gut type glucagon. More than 93% of the assayed glucagon in the effluent was eluted within the distribution of pancreatic glucagon when chromatographed under identical conditions. In fifteen experiments there was no detectable immunoreactivity corresponding to the void volume, whereas in the remaining seven experiments less than 7% was eluted at this position. The immunoreactivity in the void volume was detected variably by the four antibodies. No immunoreactivity was detected between the void volume and the elution position of pancreatic glucagon. It is concluded that the arginine stimulated porcine pancreas almost exclusively secretes one molecular species of glucagon, which probably is the 29-amino acid peptide.