An easily reproducible method was developed for isolating mice and evaluating isolation-induced aggression. A scoring schedule for rating aggressiveness and a formula to assess the dominance-subordination index (DI) were adopted. Sixty-eight white and black male mice, in 3 series, were socially isolated at 3-6 weeks of age. Increased excitability, hyperkinesia and other behavioural aberrations were observed during the first 6 weeks of isolation after which fighting sessions were started between conspecific pairs of mice. They were placed into 3 grades of aggressiveness on the basis of the scores and DI from 12 fighting sessions for each series. Aggressiveness increased till the 4th to 6th fighting session as the mice gained experience in fighting but decreased thereafter, probably, due to frequent contacts with and habituation to other mice that could be considered as 'socialization'. The respective roles of social isolation in engendering aggression and of socialization in suppressing it were discussed in relevance with the possible mechanisms.