To establish methods for stimulation of the growth and differentiation of fetal endocrine pancreatic cells, a technique for the in vitro production of fetal porcine isletlike cell clusters (ICCs) was used. By varying the composition of the culture medium with different glucose concentrations and the addition to the culture medium of insulin, growth hormone (GH), amino acids, or nicotinamide, we estimated the formation of ICCs and their hormone content. High glucose content (28.0 mM) stimulated the formation of abundant ICCs that contained decreased amounts of insulin. In contrast, culture at a low (5.6-mM) glucose concentration increased the ICC insulin content but decreased the number of ICCs formed. Addition of seven times the normal amount of amino acids hampered both the formation of ICCs and their insulin content. Neither insulin nor GH supplementation of the medium influenced the ICC insulin content, but GH stimulated an abundant outgrowth of ICCs containing relatively high insulin concentrations. However, ICCs formed under these circumstances contained less than 10% of the insulin content of adult islets, and further work has to be carried out to identify factors responsible for further differentiation of the fetal porcine pancreas.