Multi-nucleated giant cells (MGCs; Langhans-type cell), formed from macrophage fusion, are recognized as a hallmark histological feature in chronic inflammation. However, their precise pathological role is still poorly understood, especially for microorganism pathogens in the neonatal immune system, which are capable of surviving intracellularly in phagocytes. To conduct a partial evaluation of the monocyte function of neonates, we investigated the ability of human cord blood monocytes to form MGCs in vitro by stimulating various cytokines and comparing them with adult peripheral blood monocytes. Monocytes from cord blood and adult peripheral blood were isolated and cultured for 14 days with cytokines known to induce MGC in vitro. The fusion index in experiments with a combination of interleukin (IL)-4 and macrophage colony-stimulating factor (M-CSF) and a combination of IL-4 and granulocyte–macrophage colony-stimulating factor (GM-CSF) was significantly lower in cord blood than in adult blood monocytes (P = 0·0018 and P = 0·0141, respectively). The number of nuclei per MGC was significantly lower in cord blood than in adult blood monocytes in experiments with IL-4 alone, the combination of IL-4 and M-CSF, and the combination of IL-4 and GM-CSF (P < 0·0001). These results suggest the possibility that the susceptibility of newborns to mycobacterium infection is due partly to impaired MGC formation.