AbstractA 2 ✕ 2 factorial arrangement of treatments used 280 growing pigs and 240 finishing pigs to examine the main effects and interactions between two levels of stocking density (0·75 m2 v. 0·45 m2 per growing pig and 0·88 m2 v. 0·53 m2 per finishing pig) and grouping (static groups v. regrouped) on pig growth performance, carcass characteristics, nutrient digestibility and measurements of physiological indicators of stress responses. Regrouping was achieved by moving four unfamiliar pigs between replicate groups every 14 days. Average daily gain (ADG) and average daily food intake (ADFI) were reduced by an additive interaction between high density and regrouping in growing pigs (P < 0·01). In finishing pigs, regrouping reduced ADG (P 0·05) and carcass weight (P 0·05). Regrouping reduced apparent dry matter and gross energy digestibility in finishing pigs (P 0·05). Rectal temperature increased 3 h after mixing finishing pigs (P 0·05), but not growing pigs. In growing pigs, the neutrophil: lymphocyte ratio tended to increase (P 0·1) after regrouping, although plasma cortisol concentration was unaffected. In finishing pigs, the response to a Newcastle disease virus antigen challenge tended to decrease at high stocking density (P 0·1), although total IgG concentration was unaffected. In conclusion, the growth performance of growing and finishing pigs was affected by social stressors. However, effects on the physiological measurements did not concur with effects on growth performance.