A novel group of Hamadryas baboons (Papio hamadryas) containing three adult males was introduced into an already established colony of Hamadryas and hybrid baboons, with no adult males. All the unfamiliar males attacked the unweaned infants in their newly established harems. However, only one of them actually killed infants. All females in the colony who were not cycling when the union took place, including the lactating females, resumed cycling soon after the new males' arrival. In most cases lactating females had already started cycling when their infants were killed. The infants interfered with the interactions between the males and their mothers. In this way, infants initiated conflict with originally friendly males, who then started attacking the infants. Unrelated infants born shortly after the introduction developed friendly relationships with the males, in marked contrast to those born before. It was suggested that if a mother starts to develop a relationship with a new male while her infant is still dependent, the latter will respond to the male in a fearful and interfering way, possibly as a result of the decrease in maternal investment which accompanies the process of male-female bonding. On the other hand, if this bonding process is over by the time the infant is born, no infant-male conflict will arise. Young and/or subordinate males might be less capable of dealing in other ways with this kind of conflict, and therefore more prone to kill infants. Immatures followed different strategies. Oldest infants and juveniles who were not dependent on maternal care stayed away from their mothers and the new males. Younger infants who could not survive on their own actively tried to associate with other caretakers.