Two experiments were designed to test whether wild house mice discriminate between olfactory cues from different kin and, if so, whether given preferences would relate to actual reproductive decisions. Experimental animals were mice born to the offspring of wild-caught house mice. Litter-mates stayed together until 60 d of age and were then housed individually. In a choice test, animals were placed in the middle of an arena with 4 openings which led to small cages containing bedding material from opposite-sex animals of known kinship (full-sib, cousin, unrelated) or clean material. Test animals (11 oestrous females, 11 males tested with oestrous females' bedding, 8 males tested with material from non-oestrous females) preferred conspecific to control bedding. Males tested with oestrous females' bedding significantly preferred unrelated to full-sib odours. In a second experiment, 34 males were each mated simultaneously to 3 females (sister, cousin, unrelated) and these groups were then housed together for 5, 10, and 15 d. Females were checked for litters during the next 20 d. Reproductive rate increased significantly in the 15 d cohabitation group, and significantly more cousin and unrelated females than sisters gave birth to a litter.