Two P-type ATPases, MNK and WND were recently shown to be defective in the human disorders of copper transport, Menkes disease and Wilson disease respectively. These proteins are important in copper homeostasis but their full physiological function has not been established. This study uses the human breast carcinoma line, PMC42, to investigate copper transport in the mammary gland. Northern blot analysis indicated that both MNK and WND mRNA are expressed in these cells. Western blot analysis with an MNK-specific antibody demonstrated a band of approx. 178 kDa, close to the expected size of 163 kDa. Treatment of PMC42 cells with lactational hormones (oestrogen and progesterone for 3 days followed by dexamethasone, insulin and prolactin for a further 3 days) did not produce an obvious increase in MNK expression as measured by Northern and Western blots. By using indirect immunofluorescence with the MNK antibody, the intracellular distribution of MNK was found to be predominantly perinuclear, consistent with Golgi localization. Punctate staining was also seen in a smaller proportion of cells, suggesting that some MNK is associated with endosomes. Treatment of PMC42 cells with lactational hormones increased the intensity of the perinuclear and punctate fluorescence. Exposure of cells to 100 mM copper resulted in the dispersion of the fluorescence towards the periphery of the cell. The results suggest a role for MNK in the secretion of copper into milk and that PMC42 cells are a valuable model for investigating the detailed cellular function of MNK and WND.