The regulation of insulin biosynthesis, and insulin and glucagon secretion have been investigated in a human islet cell adenoma, by incubation of tumour fragments. Both biosynthesis and secretion of insulin were strongly stimulated by incubation of islet tumour cells in the presence of increasing glucose concentrations in the range 2-8 mmol/1. However, 20 mM-glucose or 20 mM-glucose plus isobutyl methylxanthine (IBMX), both of which provide potent secretagogues for normal B cells, failed to stimulate proinsulin biosynthesis and secretion from the tumour cells. Overall rates of secretion, expressed as a proportion of total insulin content, were up to 20-fold higher than those expected for normal pancreatic tissue. Glucagon secretion from the tumour was stimulated by low glucose concentrations; normal A cells also respond in this way under these conditions. However, no stimulation of glucagon secretion occurred in the presence of IBMX. There was therefore a major alteration in the regulation both of insulin and glucagon secretion, in that release of neither hormone was stimulated by cyclic AMP. Ultrastructural examination showed the tumour to be rather heterogeneous. A and B cells with normal storage granule content and structure were seen, as well as a rather larger number of B cells containing some granules of atypical appearance. The insulin content of the tumour (13 i.u./g wet wt) was consistent with 6-8% of the tumour cells being B cells.