Iron in milk and milk fractions of lactating rats, rabbits and quokkas.

-1. The iron content of whole milk and the distribution of iron between fractions separated by centrifugation was determined on milk obtained from lactating laboratory rats (Rattus norvegicus), rabbits (Oryctolagus cuniculus) and quokkas (Setonix brachyurus) at various stages of lactation. In rat milk, the distribution of 59Fe label administered in vivo or added in vitro was also studied. 2. Milk iron values previously found in rats, quokkas and rabbits by others were confirmed. In all three species the iron content of milk was highest at the beginning of lactation and was considerably reduced towards the end. In general, milk iron concentration was inversely proportional to the stage of maturity at which the young of the species are born. 3. In the rat and the quokka, the two species with particularly high milk iron content, the amount of iron in the cream (fat) and whey fractions was small but was relatively constant throughout lactation. The remainder of the iron was associated with casein; this latter fraction gave the milk its iron-rich character.