Conflicting data exist for a role for tumour necrosis factor (TNF) in memory. Here, the offspring of female mice deficient in haematopoietic TNF showed enhanced postnatal hippocampal proliferation and spatial memory in adulthood. These effects were independent of the offspring's TNF status, suggesting a maternal effect on the developing young. Indeed, TNF-deficient mothers showed low levels of various milk chemokines, and administering these chemokines to their pups restored normal hippocampal development and spatial memory. Thus, maternal TNF, through regulating milk chemokine levels, regulates hippocampal development in postnatal offspring in mice.