The mouse with X-linked anaemia [sla] has a defect in iron absorption which can be temporarily reversee by feeding a low iron diet. Duodenal non-haem iron was significantly higher in the sla than in the normal mouse on an iron supplemented diet but non-haem iron was reduced to minute amounts when the mice were fed a low iron diet. Gel chromatography on Sephadex G-200 of th partial-free supernatant of pooled mucosal homogenates revealed the presence of three proteins binding 59Fe. Fraction I [mol wt 450 000] resembled ferritin and was present in both normal and sla mice fed an iron supplemented diet. Fraction II [mol wt 78 ooo] eluted in a similar position to transferrin and was evident in both normal and sla mice fed an iron deficient diet. Fraction III [mol wt less than 15 000] contained equivalent amounts of radioiron in normal and sla mice fed the iron deficient diet, whereas this fraction contained less radioactivity in sla animals in two of three experiments in which the animals were fed an iron supplemented diet. The iron transport defect in sla mice does not appear to reside in the iron-binding proteins in the supernatant fraction of the intestinal mucosa and the cause of the defect in iron absorption remains to be elucidated.