The aim of this study was to establish an in vitro model to confirm earlier observations on the role of the myc/ras oncogenes as promoting factors in the process of normal Langerhans islet beta cell transformation. For that purpose we infected primary mouse Langerhans islets with a recombinant retrovirus containing the v-H-ras and v-myc oncogenes, before or after treatment with transforming growth factor alpha (TGFalpha). Normal Langerhans islets, when grown in culture, are viable for 2-3 weeks. After treatment with TGFalpha, viability was extended by 10 days, following which islets disintegrated. Langerhans islets transformed with v-H-ras and v-myc became immortal and insulin negative. Single infected beta cells, liberated from a primary islet into the surrounding medium, gave rise to neo islet formation. Moreover, single infected beta cells were able to grow and divide, even without fibroblast support. These results indicate that the myc and ras oncogenes are sufficient for commencement of beta cell transformation and, therefore, could represent 'early events' in the multistep carcinogenesis of insulinomas.