1. Four fitter-mates from 20 litters on each of 3 farms were divided equally and distributed at random between two treatment groups. Each piglet was injected intramuscularly with 2 c.c. of an iron preparation between 1 and 2 days of age. The control group received an iron-dextran complex which provided 150 mg. of iron per piglet and a second group received an irondextrin complex which provided 200 mg. of iron per piglet.
2. The body weight and the level of haemoglobin in the blood of each piglet were measured at 1, 8, 15, 22, and 56 days of age and the incidence and cause of disease and mortality were recorded.
3. The level of haemoglobin in the blood of both groups was comparable throughout the experiment.
4. At the 22nd and 56th day, the group receiving 200 mg. of iron was approximately 6% heavier than the group receiving 150 mg. of iron. These differences were highly significant at the 0·1% level.
5. From each of 3 additional litters, 3 piglets received no supplementary iron while 5 piglets received 200 mg. of iron provided by the iron-dextrin complex. The three whole litters were slaughtered on days 3, 8, and 22 respectively. The mean level of iron per 100 g. of liver and spleen within the untreated group on the above occasions was 5·8, 5·8, and 3·7 mg. respectively, compared with 72·9, 55·1, and 17·6 mg. respectively within the treated piglets. The differences were highly significant at 0·1 % level.