Severe aggression within groups of male laboratory mice can cause serious welfare problems. Previous experiments have shown that the transfer of specific olfactory cues during cage cleaning and the provision of nesting material decrease aggression and stress in group-housed male mice. In this study, the combined effect of these husbandry procedures was tested for their long-term effect on aggression in two strains of male mice (BALB/c and CD-1). We used postcleaning aggressive behavior, wound counts, and testosterone levels as indicators of aggressiveness. Physiological responses to social challenge were investigated through urinary corticosterone and adrenal tyrosine-hydroxylase measurements. Furthermore, the aggression-modulating effects of two enrichment items (Shepherd- Shack/DesRes and PVC tube) were explored. Marked differences were found between the two strains. CD-1 mice were more aggressive, had higher testosterone levels but lower corticosterone levels, and had fewer wounds than BALB/c mice. However, in neither of the two strains was long-term enrichment with nesting material and its transfer after cage cleaning effective in lasting reduction of intermale aggression. This may be explained by the fact that aggression levels were generally low. It seems that housing mice in small, socially stable groups or keeping social disturbances to a minimum considerably modulates aggression in group-housed male mice. Mice of both strains housed in cages enriched with nesting material had lower urinary corticosterone levels than standard-housed mice. We therefore conclude that the long-term provision of nesting material, including the transfer of nesting material during cage cleaning, may enhance the welfare of laboratory mice.