The effect of various space allowances on the pituitary, adrenal and immune responses and on performance was investigated in 72 mature Holstein x Friesian beef bulls. The animals (weighing 403+/-3.5 kg) were blocked by weight and randomly assigned to two groups (familiar, F, and unfamiliar, UF) x three treatments (1.2, 2.7 and 4.2m(2) per bull; n=24 per space allowance), and housed for 83 days in 18 pens (n=4 per pen). Blood samples were collected on days -1, 0, 3, 14, 36 and 77 with respect to mixing and housing on day 0. The bulls were given exogenous adrenocorticotrophic hormone (ACTH) on day 3 and corticotrophin-releasing hormone (CRH) on days 14, 36 and 77. Basal plasma cortisol concentration was not affected (P>0.05) by mixing F and UF bulls. On day 3, basal cortisol was greater (P<0.05) in bulls housed at 1.2 than those at 2.7 and 4.2m(2) space allowances while no effect was observed in ACTH-induced plasma cortisol concentration among treatments. Following CRH administration, there was no effect (P>0.05) of treatment and treatment x time on plasma ACTH concentration. On day 14, interferon-gamma production was lower (P<0.05) in the bulls housed at 4.2 vs. 2.7 m(2) and was intermediate but not significantly different (P>0.05) for those housed at 1.2m(2). Animals housed at either space allowances had significant (P<0.05) neutrophilia, lymphopenia, eosinopenia and decreased haemoglobin on day 3 compared with day 0. The liveweight gain from days 0 to 83 was lower (P<0.05) in bulls housed at 1.2 compared with those at 2.7 and 4.2m(2). Housing bulls at 1.2m(2) space allowance had a detrimental effect on their growth and was associated with an acute rise in plasma cortisol concentration (on day 3) compared with those having space allowances of 2.7 and 4.2m(2)/bull.