Tumour necrosis factor (TNF) has been implicated in the pathogenesis of insulin-dependent diabetes mellitus (IDDM). To investigate a possible role for TNF in IDDM we compared endogenous TNF production in two lines of non-obese diabetic (NOD) mice, NOD/Lt and NOD/WEHI, that have a high and low incidence of diabetes, respectively. Preliminary experiments had shown that the lower syngeneic mixed lymphocyte reaction (SMLR) in NOD/Lt mice could be corrected by TNF-α. Plasma TNF-αwas measured in 8 week-old female non-diabetic mice primed with 1000 units IV of murine interferon gamma (IFN-γ) followed after 3 hours by 5μmlg IV of lipopolysaccharide (LPS). Two hours later plasma was collected and TNF measured by ELISA. Plasma TNF in NOD/Lt mice was 9.2>2.4ng/ml (meanseM, n= 16) compared to 2.5>0.5 ng/ml in NOD/WEHI mice (n= 15) and 7.6>1.0ng/ml in BALB/c mice (n= 14). Time course studies demonstrated higher levels of both immunoreactive and bioactive TNF in NOD/Lt compared to NOD/WEHI mice up to 4 hours post-stimulation. A separate group of female NOD/Lt mice had IFN-γ/LPS-stimulated plasma TNF-αmeasured at 10 weeks and were followed to age 30 weeks. The mean stimulated plasma TNF-αlevel wsa consistently higher in those mice that developed diabetes compared to those that remained non-diabetic, the difference being significant when mice were 21 weeks of age. These results suggest that endogenous TNF-αproduction may be a trait marker of IDDM susceptibility.