Abstract Alkali treatment of fully cooked soya flour was found to increase the performance of many young animals, but few experiments have been carried out with pigs. We have investigated the effect of an alkali treatment of soya products on some biochemical parameters of protein in vitro and on the performance of young piglets. With a soya flour and 2 soya-bean meals, all well toasted and with very low anti-trypsin activity, alkali treatment considerably improved protein breakdown by trypsin in vitro, while protein breakdown by pepsin was hardly affected by the treatments. In 3 growth trials with piglets weaned at 3 weeks, it was invariably shown that those given alkali-treated soya flour as the only protein source performed significantly better than animals given the same diets but with untreated soya flour. A digestibility trial showed no difference between these diets, but nitrogen balance was 10% higher ( P < 0.005) with alkali-treated soya flour. Although the improved nitrogen retention would suggest an increased bioavailability of some amino acids in the alkali-treated soya flour, the results are not in contradiction with the hypothesis of a “hidden” or “bound” trypsin inhibitor and/or other anti-nutritional factor(s) still present in properly heated soya flour. These anti-nutritional factors might be more important in young than in older animals.