A programme of study was undertaken to assess the effect of pretreating raw soya bean and processed full fat soyabean meals with protease enzymes prior to use in liquid feed for grower and finisher pigs. A series of laboratory studies was undertaken to examine the efficacy of three microbial proteases (P2, P3 and P4) in partially hydrolysing soya protein and in reducing the levels of trypsin inhibitors in raw soyabean. Pretreatment consisted of steeping ground soyabean for 24 h at 20° C in the absence (control) or presence of 20 000 units gˉ¹ N of P2, P3 or P4. Pretreating raw soyabean (RSB) with P2, P3 and P4 significantly (P < 0.05) reduced trypsin inhibitor levels from 28.53 to 19.98, 17.17 and 18.35 (s.e.m.1.14) mg trypsin inhibited gˉ¹ soya respectively. Pretreating RSB, micronized (MIC) or autoclaved (AUT) soyabean meal with P2, P3 or P4 resulted in increases in soluble a.-amino nitrogen of 5.22, 7.08, and 6.58 (RSB), 5.11, 5.57 and 4.32 (MIC) and 3.56, 7.03 and 6.18 (s.e.d. 0.06) mg gˉ¹ soya respectively and in vitro digestibility of nitrogen of 7.6 %, 9.9 %and 6.4 % (RSB), 4.9 %, 8.3 % and 2.8 % (MIC) and 11 %, 8 % and 12.2 % (AUT) respectively compared with the appropriate controls. Feeding trials were conducted in which pretreated soya was added to a basal cereal diet. Pretreatment of RSB with P4 resulted in a significant (P < 0.05) increase in ADG of 0.08 kg pigˉ¹ dˉ¹ (s.e.d. 0.04) in grower pigs (33.5 ± 4 kg) over the 6 week duration of the trial but had no significant effect on FCR. Pretreatment of AUT with P4 did not significantly improve performance. In a feeding trial with grower/finisher pigs pretreatment of RSB with P3 resulted in significant (P < 0.05) improvements of 0.10 (s.e.d. 0.04) kg pigˉ¹ dˉ¹ in ADG and 0.476 (s.e.d. 0.19) in FCR. Pretreatment of MIC with P3 resulted in a significant reduction (P < 0.05) of 4 d (s.e.d. 1. 7) in the time taken for pigs to attain slaughter weight.