Male and female Sprague-Dawley rats were fed purified diets in which the carbohydrate component was either starch or refined sugar (sucrose). The addition to these diets of the ash prepared by the incineration of unrefined muscovado sugar prevented the deficiencies of Factor R seen in the offspring when the diets were not supplemented with ash. Analysis by neutron activation showed that the ash from the unrefined sugar significantly increased the proportion of iron, cobalt, manganese, caesium and rubidium in the diets. The addition of chlorides of all five mineral elements to the diet containing refined sugar also prevented the development of signs of deficiency of Factor R in the pups. However the addition of cobalt chloride alone, or of cobalt and manganese chlorides, did not prevent the deficiency. It is likely that what we have called reproductive Factor R is iron, caesium or rubidium.