Cells infiltrating the Langerhans' islets of prediabetic NOD females were isolated from 6 weeks to 6 months of age. These cells were assayed at a single-cell level for production of eight different cytokines by intracellular immunofluorescent staining. Quiescent in vivo preactivated cells were detected by in vitro stimulation with PMA and ionomycin for 4 h. The cell recruitment phase, between 6 and 12 weeks of age, is predominated by production of the monokines IL-1alpha, IL-6, and TNF After stimulation IFN-gamma and occasional IL-10 and GM-CSF producing cells could also be observed. This cytokine pattern occurs simultaneously with increasing insulitis, and we suggest that these cytokines are important in attracting inflammatory cells to the islets and maintaining the inflammatory state. A high frequency of endocrine cells producing IL-6 during this period may denote a stress response caused by initial beta-cell destruction due to cytokines released by the inflammatory cells. During the effector phase, between 4 and 6 months, there is a characteristic Th1 cytokine profile with lymphocytes producing IL-2, IFN-gamma and TNF, supposedly TNF-beta. No IL-4 production could be detected and IL-10 was very rarely found, indicating the absence of a Th2 response. Our findings show that the effector phase in NOD insulitis is a Th1 rather than a Th2-mediated event. We also demonstrate that cytokines that may cause initial tissue destruction are produced during the recruitment of inflammatory cells.