The behaviour of lactating house mice (Mus domesticus) with litters of young of different degrees of relatedness was analyzed under different environmental conditions to test whether females recognize and discriminate in their maternal behaviour between own and alien young. Both under ad libitum and under restricted feeding there was no significant difference in the amount of time a female spent nursing or licking own versus alien young (litters consisted of four own and four unrelated alien pups, cross-fostered on day 1 of lactation). Weaning weight was also not significantly different. Under restricted feeding the females reduced their litter sizes by killing some young. On average five out of eight pups survived until weaning. The females killed as many own as alien pups. In a third series of experiments two females (either familiar sisters or unrelated, unfamiliar females) were allowed to share a communal nest and they always indiscriminately nursed both litters (food was available ad libitum). However, the degree of relatedness of the females sharing a communal nest did affect their lifetime reproductive success (measured as the number of offspring weaned during a lifetime of 6 months): a female sharing a nest with a sister weaned significantly more young than a monogamously paired female; the reproductive success of a female living with an unrelated partner did not differ significantly from that of a monogamous female or from that of a female living with a sister. Although females did not discriminate in their maternal behaviour between own and alien young in the same nest, kin recognition might be important for choosing a partner for cooperative brood care.