‘Environmental enrichment’ is often considered to improve captive animal welfare. However, some studies using male mice, Mus musculus, indicate that increasing cage complexity increases aggression. Limited evidence suggests that enrichment differs in its effects on behaviour and physiology between strains; but behaviour also differs between strains in non-enriched environments. Differences in enrichment type, evaluation methods, and strains used, have caused difficulty in interpreting the efficacy of environmental enrichment in improving welfare. Using enrichment suitable for commercial laboratories (nesting material and a Perspex tunnel), we compared within-cage behavioural and physiological responses among males of six strains housed in non-enriched standard polypropylene cages with those housed in ‘enriched’ cages. Outbred ICR(CD-l) and TO mice, and inbred BALB/c mice were more aggressive than C57BL/6, CBA/Ca and DBA/2 mice, which exhibited low levels of aggression typical of most inbred strains. Enrichment did not significantly affect aggression levels. Animals in enriched cages spent more time investigating the internal cage environment, eating and drinking, and in stereotypic behaviour patterns, although levels differed between strains. The greatest increase in stereotypy levels (bar-related stereotypies) with enrichment was found in DBA/2 mice. Higher testosterone levels were maintained over the study period in mice housed in enriched cages, and in more aggressive strains. IgG levels were also higher in mice housed in enriched cages, and in the outbred strains ICR(CD-l) and TO compared with inbred strains. The relationship between aggression, testosterone and ‘enrichment ‘ suggests that increasing complexity in laboratory cages may increase a naturally selected territorial response in some strains. The implications for strain-specific welfare are discussed.