AIMS/HYPOTHESIS: The neogenesis of islets from cultured human adult pancreatic tissue has been reported. The islet progenitors have been thought to be ductal cells. Since previous experiments have been 'contaminated' by a number of pre-existing islet cells, we examined their involvement in islet cell neogenesis. METHODS: Fresh human pancreatic cells with different purities of islet cells were grown in monolayer culture and labelled with bromodeoxyuridine. Transitional cells were analysed by double immunofluorescence staining. For purified ductal cell culture, pre-existing islets were eliminated on a magnetic cell separation system. RESULTS: We confirmed that less than 1% of the endocrine cells proliferated, mainly during the first 48 h of culture. However, a 10-fold larger proportion of the cells acquired a transitional phenotype by starting to coexpress the ductal marker cytokeratin 19 (CK19). These cells represented more than 10% of all endocrine cells after 1 day in culture, and 6% at 5 days of culture. Using magnetic cell sorting, we eliminated cells expressing neural cell adhesion molecule (N-CAM), after which we obtained 99.7% pure non-endocrine CK19-rich cell populations. These cell populations could be expanded in vitro. However, their endocrine differentiation capacity was severely reduced as compared with the original mixed cell cultures. CONCLUSIONS/INTERPRETATION: These results suggest that islet neogenesis in this culture system at least partly represents the de-differentiation of islet cells into a duct-cell-like phenotype, with further re-differentiation in appropriate conditions. The plasticity of differentiated human pancreatic cell types may thus be an important mechanism of human pancreas regeneration.