Recently mated female house mice separated from their mates and exposed to unfamiliar (strange) males often undergo pregnancy disruption. A favored explanation for this phenomenon, known as pregnancy block, is that it has evolved as a female counterresponse to potential infanticide by males. Curiously, pregnancy block is believed not to occur during lactational pregnancy, even though that lactational pregnancy is thought to be common in house mice. We examined the potential for male-induced pregnancy block to occur during lactational pregnancy in female mice after lactation disruption by both male infanticide (Expt 1) and litter removal (Expt 2). Strange males were paired with females that had both recently given birth to a first litter and potentially been reimpregnanted by the original stud male during postpartum estrus. Hence, some of the females' second litters were sired by the stud males and pregnancy block could then be detected as a drop in the delivery rate of such litters among these experimental females relative to control females that were not exposed to strange males. Pairs remained housed together until after a second litter was born. Thus, we were also able to measure the influence of prolonged heterosexual cohabitation on infanticide by strange males towards subsequent litters. If, as some have found, non-sexual contact with a pregnant female suppresses male infanticide, then the importance of male infanticide as an explanation for pregnancy block is further drawn into question. When strange males were introduced to lactating dams approximately one and a half days postpartum (Expt 1), male infanticide effectively halted lactation. However, none of these females subsequently showed pregnancy block. In contrast, when strange males were introduced to dams approximately one day postpartum and lactation was interrupted even earlier (Expt 2), pregnancy block did occur. There was no evidence for a cohabitation-induced reduction of male infanticide in either experiment; most strange males initially categorized as infanticidal subsequently killed females' second litters, provided they had not sired those litters. Despite the lack of cohabitation effects on male infanticide, because pregnancy block may only occur under restricted conditions during lactational pregnancy, the role of male infanticide in shaping the evolution of pregnancy block may be less straightforward than previously thought.