Human pancreatic beta-cell proliferation may be crucial for the success of islet transplantation. The aim of this study was to test the hypothesis that adult human beta-cells proliferate in vitro and in vivo and respond with increased rates of replication to factors known to promote rodent islet-cell proliferation, i.e. glucose, human recombinant GH, and FCS. For this purpose, human islets were prepared from a total of 19 adult heart-beating organ donors and cultured for 48 h with or without the additives described above. 3H-thymidine was added to the medium during the last 60 min of culture. After immunohistochemical staining for insulin and autoradiography, the labeling index (LI; i.e. % of labeled beta-cells over total number of beta-cells) was estimated by light microscopy. Islets also were transplanted under the kidney capsule of normal or alloxan-diabetic nude mice. After 2 weeks, 3H-thymidine was injected and the islet grafts prepared for determination of LI, as described above. Islets cultured at 5.6 mM glucose showed an increased beta-cell proliferation compared with islets cultured at 2.8 mM glucose (P < 0.05). However, culture at 11 mM glucose failed to further increase beta-cell proliferation. Addition of GH (1 microg/ml) to the medium, in the presence of 1% FCS and 5.6 mM glucose, did not influence the rate of beta-cell proliferation. In islets transplanted to hyperglycemic nude mice, beta-cell proliferation was similar to that observed in islets grafted into normoglycemic nude mice. Proliferation, however, decreased with increasing organ donor age. This study shows that pancreatic beta-cells from adult man are able to proliferate both in vitro and in vivo. Moreover, beta-cells from adult human donors respond with increased proliferation to glucose in vitro and show a decreased proliferation in vivo with increasing donor age.