1. A trial was carried out with 96 purebred Large White and Landrace pigs to assess the optimum content of skimmed milk in diets given from 20 to 90 kg.
2. Pigs on treatment 1 (control) received a barley and soya bean diet providing 12·9 MJ digestible energy per kg and 161 g crude protein per kg until they reached 54 kg and 144 g crude protein per kg thereafter. Those on treatments 2 to 4 received 3·5, 4·5 and 5·75 1 skimmed milk per day, respectively, from 28 kg live weight, after a gradual increase from 2·0 I/day at 20 kg; this was given with a basal barley diet which provided 12·9 MJ digestible energy per kg and 111 g crude protein per kg. All diets were given according to scales to permit all pigs equal daily intakes of digestible energy. Treatments 1 and 2 were approximately isonitrogenous.
3. In order of treatment, average daily gains (g) were 658, 712, 743 and 737 (s.e. of difference, 15) and food: gain ratios were 3·06, 2·73, 2·63 and 2·60 (s.e. of difference, 0·05). There were no important treatment effects on linear carcass measurements.
4. It appeared that lysine supply was a major determinant of the results obtained.
5. The greatest daily gains were achieved when a minimum of 4·5 1 of skimmed milk was given daily with the basal diet; this approximates to an average skimmed milk: meal ratio of 2·8 I/kg.