1. In separate experiments the isoleucine and leucine requirements of the kitten were determined on the basis of growth and nitrogen retention. The dietary concentrations of isoleucine tested were (g/kg diet) 1.4, 2.2, 3.0, 3.8, 4.6 and 9.0 with adequate (12.0 g/kg diet) leucine. The levels of leucine tested were (g/kg diet) 5.0, 7.5, 9.0, 10.5, 12.0 and 20.0 in diets containing adequate (9.0 g/kg diet) isoleucine. In both experiments six male and six female kittens received each dietary level of isoleucine or leucine for periods of 10 d in a balanced 6 x 6 Latin-square experimental design.
2. Asymptotic curves were fitted to the response relationships and the minimal dietary requirements for maximal response were estimated from the values at 0.95 of the asymptote. On this basis, the requirements for maximal growth were 6.2 g isoleucine/kg and 7.8 g leucine/kg diet. The requirements for maximal N retention were higher; 8.4 g isoleucine and 10.6 g leucine/kg diet. The isoleucine requirements suggested by this method are probably overestimations and might be slightly above 4.6 g/kg diet.
3. Plasma isoleucine and leucine concentrations were not useful in estimating the requirements. Plasma leucine increased rectilinearly with increasing dietary leucine while the response of plasma isoleucine to increasing dietary isoleucine was non-rectilinear. Neither response relationship exhibited a breakpoint at the level of requirement. Below the suggested minimal requirement for leucine there were significant increases in the concentrations of isoleucine and valine in the plasma. Dietary isoleucine below the level of requirement had no effect on plasma valine and leucine. Dietary leucine had no effect on the plasma concentrations of methionine, phenylalanine and threonine, suggesting that the effect of decreasing dietary leucine on plasma isoleucine and valine is a result of decreased oxidation rather than decreased protein anabolism.
4. In a separate experiment six kittens, presented a diet containing 2.2 g isoleucine/kg, developed crusty exudates around their eyes within 27 d and six kittens, presented diets containing 3.8 g isoleucine/kg, showed this clinical sign but with less severity within 47 d. Cultures of conjunctival swabs taken from the most severely affected kittens showed the presence of staphylococcal species, suggesting that in isoleucine-deficient kittens there was impaired resistance to these dermal microbes.