(a,b) Male and female mice were tested for locomotor activity in the open field and for anxiety-like behaviour by using the elevated plus maze (a; n=72 (males, n=11 CT, 12 AB, 6 AB/JB1: females, n=17 CT, 13 AB, 13 AB/JB1), (b); n=71 (males, n=10 CT, 12 AB, 6 AB/JB1: females, n=17 CT, 13 AB, 13 AB/JB1)) (two-way ANOVA). (c,d). Social behaviour and preference for social novelty were tested in the three-chambered apparatus (n=69; males, n=11 CT, 12 AB, 6 AB/JB1: females, n=16 CT, 12 AB, 12 AB/JB1) (two-way ANOVA). (e–g) Males (n=11 CT, 12 AB, 5 AB/JB1) were subjected to microdefeat and tested 24 h later for social avoidance. Results are means±s.e.m. (except for graph F where results are means±s.d.). *P<0.05, **P<0.01, ***P<0.001. AB, antibiotic; AB/JB1, antibiotic and L. rhamnosus JB-1; CT, control.