The changes in cellular composition of the milk of the koala (Phascolarctos cinereus) have been investigated in late lactation and after the loss of pouch and back young. During lactation, the polymorphonuclear neutrophil was the most frequently observed cell. After loss or removal of the young, macrophages and foamy epithelial cells were detected in moderate to high numbers. In one animal, mammary secretions containing lymphocytes and unidentifiable, immature mononuclear cells continued for 120 days after the loss of pouch young. These observations of cellular composition of late- and post- lactation secretions are consistent with reports from a number of eutherian mammals and also with another marsupial, the tammar wallaby (Macropus eugenii).