This brief review examines the roles of sex steroids in two forms of "aggression" in laboratory mice. The social conflict induced in male mice by individual housing or reproductive experience was contrasted with the attack shown by small groups of female mice on lactating intruders. Gonadectomy of males largely abolishes the former response but actually augments the latter activity in male subjects. Testosterone, estradiol and 5 alpha-dihydrotestosterone all restore social conflict in gonadectomized male mice but these sex steroids inhibit attack on lactating female intruders by gonadectomized males. The data clearly confirm that there is no simple relationship between a particular hormone and "aggression." These forms of attack may serve very different functions even though they involve similar action patterns and distributions of bites on the attacked animal. A tentative discussion is included about the roles of these activities.