There is circumstantial evidence of a causal connection between type 1 diabetes in young Icelandic males and consumption of smoked cured mutton, containing N-nitroso compounds, by their parents at about the time of conception. This hypothesis has been examined in CD1 mice, and such processed mutton, consumed by the parents before mating and during pregnancy and by the offspring from day 19 until study 1-5 weeks later, produced diabetes in just over 16% of male progeny and 4.2% of female progeny. Light and electron-microscopic changes in the beta-cells were those of stages of cell death. The parent mice showed no features of diabetes. When both parents were fed the Icelandic cured mutton only up to the time of fertilisation, there was first a fall and then a significant rise in the plasma glucose of the male progeny after the 3rd week of age. The female progeny showed a significant fall in plasma glucose at 5-6 weeks of age. These findings suggest that an environmental factor in the aetiology of human diabetes mellitus had been identified. The mechanism seems to involve parental as well as maternal influences of germ cells.