1. One hundred and sixty-eight pigs were used in four experiments to test the value of including antibiotics and copper sulphate, either as separate supplements or together in rations for growing pigs. Procaine penicillin was added at the rate of 5·36 mg./lb. diet and Aureomycin (in Aurofac 2 A) at 8·03mg/lb. and CuSO4. 5H2O as 0·1% of the diet.

2. During the growing period in Exp. 1, pigs fed diets supplemented with copper sulphate or procaine penicillin grew 9 and 3% faster respectively than the controls. Pigs fed a diet containing both supplements grew 22% faster than the controls. During the finishing period there was an outbreak of virus pneumonia and the within-treatment variability was large. There were no significant differences between treatments during this period, although pigs fed the supplemented diets tended to grow more slowly than the controls. Over the total experimental period pigs fed the diet supplemented only with copper sulphate grow 2% faster than the controls, and those fed the diet supplemented with both procaine penicillin and copper sulphate grew 4% faster than the controls, but these small average improvements were not statistically significant. In Exp. 1 treatment differences in food conversion efficiencies paralleled those for growth rates. Killing-out percentages were higher when pigs were fed the copper-supplemented diets, and lower when procaine penicillin was fed. The area of ‘eye’ muscle was increased by adding procaine penicillin to the diet.

3. During the growing period in Exp. 2, pigs fed diets supplemented with copper sulphate or with both procaine penicillin and copper sulphate grew 8 and 9% faster, respectively, than the controls. The addition of procaine penicillin had no significant effect when added as a single supplement or in conjunction with copper sulphate. During the finishing period pigs fed diets supplemented with copper sulphate, procaine penicillin, or both copper sulphate and procaine penicillin grew 5, 3 and 10% faster, respectively, than the controls, while over the total experimental period they grew 4, 0 and 9% faster, respectively.

4. Some pigs fed the copper-supplemented diets during the growing period in Exp. 2 were fed diets with no copper added over the finishing period. During this latter period the pigs grew at about the same speed as the controls, and significantly more slowly than those pigs which continued to receive copper sulphate. Thus, the average growth rates over the total experimental period were significantly slower by 3–7% when copper was omitted after 100 lb. than when it was fed to the pigs throughout the entire experiment.

5. In Exp. 2, as in Exp. 1, treatment differences in efficiency of food conversion paralleled those for growth rates. A higher rate of growth was always accompanied by an improvement in efficiency of food conversion, with about the same percentage difference between treatments. Indeed, changes in rate of growth were probably dependent upon changes in efficiency of food conversion, since all pigs were fed to the same scale based on live weight.

6. The treatment differences in killing-out percentage and area of eye muscle observed in Exp. 1 were not confirmed in Exp. 2. There were no differences in carcass quality measurements attributable to the treatments imposed in Exp. 2.

7. The livers of pigs fed diets containing copper sulphate during both the growing and finishing periods had, on average, about eighteen times the concentration of copper found in the controls. When the copper sulphate supplement was fed only during the growing period the liver copper concentrations were, on average, about four times those of the controls. However, the variation between the individual values was very large in all groups fed copper sulphate. This variation may have been caused partly by sampling errors if copper is not distributed evenly throughout the liver.