Insulin release from fetal beta-cells responds only minimally to acute glucose stimulation. Glucagon-like peptide-1 (GLP-1) is able to correct glucose sensitivity in some models of glucose-resistant beta-cells. We have now tested whether GLP-1 can induce glucose-responsive insulin release in human fetal islet-like cell clusters (ICCs). For this purpose we used perifusion and static incubation of ICCs and isolated adult human islets. In perifusion, the fetal ICCs responded with only minimal insulin release to 16.7 mmol/L glucose or 10 nmol/L GLP-1 combined with 1.67 mmol/L glucose. However, in the presence of GLP-1, the insulin response to glucose was markedly potentiated and appeared in a biphasic manner (10-fold first phase and 3-fold second phase). The first phase response was equal to that of adult human islets in similar experiments, whereas the second phase of the fetal cells was significantly lower. In static incubations, the relative insulin release responses to 16.7 mmol/L glucose plus 10 nmol/L GLP-1 were equal in the fetal and adult cells. The cAMP content of the cells was increased by glucose only in the presence of GLP-1. Our studies indicate that the glucoincretin hormone GLP-1 is able to constitute biphasic insulin release in the immature beta-cell, possibly as the result of cAMP-mediated priming of the glucose sensor mechanism.