In leukemic strains of mice, lymphoid leukemia occurs less often and later in males than females. However, in author''s AK mice, males that were isolated early in life and did not fight died of spontaneous leukemia at essentially the same age and in the same proportion as females, whereas fighting males died significantly later and in a lower percentage, whether losing or winning the fights. Hence the difference between sexes is not explained by sex hormones only. An interpretation is proposed, based on adrenal corticosteroid secretion. Fighting activity should be taken into account as a factor in pathogenesis of mouse leukemia and in the difference of incidence and survival time between male and female mice.