To investigate qualitative and quantitative differences in islet cell antibody (ICA) detection within and between species we evaluated 8 pancreatic substrates (2 human, 2 baboon, 2 macaque, 1 pig and 1 rat). All substrates were able to detect strongly positive sera (with ICA greater than 160 JDF units). Non-human primate pancreata showed a larger size and greater abundance of islets than human, pig or rat. Significant differences in end point titter were observed both within and between species. In comparing the two best pancreata (1 human and 1 macaque) in masked screening of 50 normal controls and 108 nondiabetic first degree relatives of type 1 diabetics, very similar results were obtained. We concluded that pancreatic substrates from a variety of species may be useful for detection of ICA. This study, however, underscores the need for between and within assay standardization of ICA such as those currently underway in the International Immunology of Diabetes Workshops and Proficiency Tests.