Weaning often combines several stressful events such as sudden change of diet and mixing with unfamiliar piglets. These sudden changes induce behavioural, endocrine and immune alterations and are followed by a transient reduction of growth and digestive disorders. At weaning, the physiological effects of social reorganization could be masked by the much more important effects of the diet change. The aim of this study was to evaluate the reactivity of piglets to mixing by dissociating social reorganization from weaning itself. This study investigated the influence of mixing five days after weaning, when most of the acute physiological adjustments of weaning have disappeared. The response to mixing was assessed by measuring salivary cortisol, blood lymphocyte proliferation, interferon- production and behavioural activity. Cortisol levels were increased after mixing and returned to basal values within 24h and the blood lymphocyte level was not affected by mixing. Mixing increased resting behaviour. Piglets seemed to avoid conflicting encounters by diminishing the synchronization of their activities with their new group. The results of this study show that mixing young piglets is stressful but that animals develop behavioural strategies to adapt to the situation.