The objective of this study was to examine the effect of abrupt weaning (inclusive of social group disruption and maternal separation) on the physiological mediators of stress and measures of immune function. Thirty-six male and 36 female calves (Limousin and Charolais crosses), habituated to handling, were blocked by sex, weight, and breed of dam and randomly assigned, within block, to either a control or abruptly weaned group. Animals were separated into the respective treatment groups at 0 h. Calves were bled at -168, 6 (males only), 24, 48, and 168 h after weaning, and the behavioral reaction of calves to handling was scored. Cortisol, catecholamine (not sampled at -168 h), acute-phase protein concentrations, and in vitro interferon-gamma production and neutrophil:lymphocyte ratio were measured. The effects of weaning, calf sex, time, and their respective interactions were described. Disruption of the established social group at 0 h increased (P < 0.001) the plasma cortisol concentration and neutrophil:lymphocyte ratio and decreased the leukocyte concentration (P < 0.001) and the in vitro interferon-gamma response to the mitogen concanavalin-A (P < 0.001) and keyhole limpet hemocyanin (P < 0.001) for weaned and control animals compared with -168 h. There was no effect of weaning or sex on the behavioral reaction of calves to handling. Plasma cortisol and adrenaline concentrations were not affected by weaning or sex. Plasma noradrenaline concentration was influenced by weaning x sex (P < 0.05) and time x sex (P < 0.05). The response increased for male calves with weaning and increased with each sampling time after weaning. For heifers, the response was not affected by weaning and plasma concentrations decreased at 168 h after weaning. There was no effect of weaning or sex on leukocyte concentration. The neutrophil:lymphocyte ratio increased after weaning (P < 0.01) and was affected by sex (P < 0.05). Weaning decreased (P < 0.05) the in vitro interferon-gamma response to the antigen keyhole limpet hemocyanin. There was a time x weaning x sex (P < 0.05) interaction for fibrinogen concentration but no effect of treatment on haptoglobin concentration. Abrupt weaning increased plasma cortisol and noradrenaline concentrations that were accompanied by attenuation of in vitro interferon-gamma production to novel mitogen and antigen complexes up to 7 d after weaning.