The non-obese diabetic (NOD) mouse is an animal model of human insulin dependent diabetes mellitus (IDDM). In this strain, the serum concentration of tumor necrosis factor-alpha (TNF alpha) after OK432 (a streptococcal preparation) stimulation is much lower than in any other non-diabetic control strain. Female NOD mice which have a higher incidence of diabetes have significantly lower TNF alpha level (6.5 +/- 4 U/ml, mean +/- SEM) than do male NOD mice (21 +/- 5 U/ml) (P < 0.02) which have lower incidence of diabetes. On the basis of these results, we designed a prospective study to evaluate the relationship between the serum TNF alpha concentration and the incidence of diabetes in individual male NOD mice. Mice were studied until 30 weeks of age. During this period four of eight mice with a low TNF alpha level (TNF alpha < or = 1.1 U/ml) became diabetic, whereas none of eighteen mice with a high TNF alpha level (TNF alpha > 1.1 U/ml) developed overt diabetes. These results indicate that by measuring of endogeneous TNF alpha level after stimulation by OK432, one could predict IDDM in male NOD mice.