The feasibility of somatic cell gene therapy as a method of insulin delivery has been studied in mice. Murine pituitary AtT20 cells were transfected with a human preproinsulin DNA in a plasmid containing a metallothionein promoter and a gene conferring resistance to the antibiotic G418. The AtT20MtIns-1.4 clone of cells was selected because of its higher insulin-releasing activity compared with other clones. After culturing for 24 h in Dulbecco's medium containing 10 mM glucose, the AtT20MtIns-1.4 cells released human insulin at about 5 ng/10(6) cells per 24 h. Insulin release was not significantly altered by raised concentrations of glucose, potassium or calcium, but insulin release was increased by 20 mM arginine, 5 mM isomethylbutylxanthine and 90 microM zinc. AtT20MtIns-1.4 cells (2 x 10(6)) were implanted intraperitoneally into non-diabetic athymic nude (nu/nu) mice, and the mice were made diabetic by injection of streptozotocin after 7 days. Release of human insulin in vivo was assessed using a specific plasma human C-peptide assay. Human C-peptide concentrations were maintained at about 0.1 pmol/ml throughout the 29 days of the study. The development of streptozotocin-induced hyperglycaemia was delayed in recipients of the cells releasing human insulin, compared with a control group receiving an implant of non-transfected cells. At autopsy the implanted AtT20MtIns-1.4 cells in each recipient had formed a tumour-like aggregation, with an outer region of insulin-containing cells.(ABSTRACT TRUNCATED AT 250 WORDS)