1. Pancreatic islet homogenates catalyze, in a Ca2+-dependent fashion, the incorporation of [2,5-3H]histamine, [1,4-14C]putrescine, [1,2-3H]agmatine, [14C]methylamine and L-[U-14C]lysine in N,N-dimethylcasein. 2. Using [2,5-3H]histamine as the amine donor, the Km for Ca2+ and histamine amounts to 90 microM and 0.7 mM, respectively. 3. The incorporation of [2,5-3H]histamine into N,N-dimethylcasein is inhibited by monodansylcadaverine, N-p-tosyl glycine, bacitracin and methylamine, the relative extent of inhibition depending on the respective concentrations of Ca2+, inhibitor and amine donor. 4. Bacitracin and methylamine, but not N-p-tosyl glycine, cause a dose-related inhibition of glucose-stimulated insulin release. 5. It is concluded that, in pancreatic islets, the Ca2+-responsive transglutaminase activity plays a critical role in the process of glucose-induced insulin release.