Male mice tend to be infanticidal to unrelated infants but parental to their own offspring. The present study examines three hypotheses that might explain this apparent discrimination. There was no evidence of selective infanticide toward infants encountered for the first time on the basis of kinship, location, or cues associated with previous sexual partners. However, males tended to direct more paternal responses toward their own unfamiliar infants than toward unrelated infants, infants encountered in the male's home cage than those in the cage of another male, and when cues associated with a previous sexual partner were present. Data suggested that the responses of females to male intruders might influence the responses of those males. It was concluded that the infanticidal responses of male mice are mediated by a particular state of the male but that males in a noninfanticidal state may vary their paternal responsiveness on the basis of direct and indirect cues concerning relatedness.