Subtotal pancreatectomy specimens from one case of nesidioblastosis, one case of focal adenomatosis, and two cases of insulin-producing islet-cell tumours were studied with special reference to their production of pro-insulin, C-peptide and insulin, and their contents of argyrophil parenchymal cells. Specific immunostaining revealed the presence of abundant cells reacting with pro-insulin, C-peptide, and insulin antiserum; at least the great majority of them were obviously non-argyrophil cells. The content of extractable immunoreactive insulin (IRI) was higher in the cases of nesidioblastosis and focal adenomatosis than in the two insulomas. Molar ratios of IRI to C-peptide immunoreactivity (CPR) varied between 7 and 100. Gel filtration analysis of the extracts revealed two peaks of CPR, corresponding to 3,000 and 10,000 daltons, respectively. Ultrastructurally, the insulin cells in cases of nesidioblastosis and focal adenomatosis contained numerous typical beta granules. In the islet-cell neoplasms some "polycrine" islet cells were also found, containing typical as well as atypical granules with electron dense or pale cores. Some cells even showed a mixture of apparent beta and alpha granules. Despite structural differences and variable contents of IRI and CPR, the predominance of cells reactive with antibodies to pro-insulin, C-peptide, and insulin, and the absence of argyrophil pro-insulin cells in adenomatosis and insulomas indicates that the hormonal products of these parenchymal cells are not any chemically modified insulin or any other member of the insulin family.