1. Thirty-three diets were made by adding to ground barley combinations of L-lysine (0--6.0 g/kg) and L-threonine (0--3.0 g/kg), together with vitamins and minerals. Each was given to two female and two castrated male pigs during their growth from 25 to 60 kg. 2. Growth rate increased from 0.36 kg/d to a maximum of 0.65 kg/d with additions of 3.8 g L-lysine/kg and 1.8 g L-threonine/kg; these values and the maximum gains achieved were slightly higher for castrates than for females. Minimum values for food conversion ratio of 2.84 for castrates and 3.06 for females were achieved with similar amino acid additions to those giving fastest growth. 3. Carcass fat, estimated by sepcific gravity, was least with the addition of 5.9 g L-lysine/kg, but continued to decrease up to the highest threonine concentrations. Backfat thickness was also reduced by amino acid additions but failed to reach a minimum.