Cytokines secreted by antigen presenting cells, lymphocytes T and pancreatic beta cells are considered as the major mediators in the pathogenesis of IDDM. It has been suggested that cytokines released by macrophages/monocytes could have an initial role in beta-cell damage. The aim of the present study was the estimation of in vitro production of macrophage-derived cytokines: IL-1 beta, TNF-alpha, IL-12 by peripheral blood in high risk IDDM first degree relatives, since it could reflect early events leading to the development of type 1 diabetes in humans. The study was performed in 25 high risk IDDM subjects and 21 age and sex-matched healthy controls. IL-1 beta, TNF-alpha and IL-12 concentrations in supernatants of whole blood cultures with PHA (10 micrograms/ml) were quantified by ELISA. In the ICA positive relatives of IDDM subjects levels of IL-12 were significantly higher as compared with the control group, both at 48 h (p < 0.02) and at 72 h (p < 0.05) of incubation and positively correlated with TNF-alpha and IL-1 beta (R = 0.46, p < 0.02 and R = 0.32, p < 0.05). We did not observe statistical differences in in vitro production of TNF-alpha and IL-1 beta between the study groups. In conclusion we suggest that our findings support the hypothesis, that IL-12 is involved in the pathogenesis of human IDDM. If the involvement of Th1 cells is confirmed in the destruction of islet beta-cells, it is possible that IL-12 antagonists will have a role in the future prevention of insulin dependent diabetes mellitus.