Eight diets based on barley and soya bean meal were formulated. Two basal diets, one of which included fish meal, were formulated to conform to the recommendations of the Agricultural Research Council (1981) for all amino acids except lysine. Four further diets were made by adding L-lysine to these diets, in two increments; the highest concentrations of lysine were also supplied in two further diets by the inclusion of a higher proportion of soya bean meal. Each diet was given ad libitum to 10 growing pigs, five castrated males and five females, which were housed and fed individually.

Measurements of performance were made over the weight range 22 to 55 kg. Carcasses were appraised after slaughter at a mean weight of 75 kg.

Addition of lysine, but not of soya bean meal, significantly increased daily food intake and carcass fatness. Daily growth and food conversion efficiency were not significantly affected. Because of the high food intake the daily lysine intakes on the basal diets greatly exceeded the suggested daily requirement and the results indicate that in such circumstances the response to the amino acid supplementation of an apparently inadequate protein may be negligible.