Tumour tissues are frequently seen with monocyte/macrophage infiltration. This study was to establish the role of monocytes on the motile and invasive behaviour of human cancer cells. By using a co-culture technique, we have shown that both human peripheral blood monocytes and a monocytic cell line, U937, stimulated colon and breast cancer cell colony scattering, motility, and invasion into a basement membrane (Matrigel). This effect was enhanced when monocytic cells were stimulated by a particulate stimulus. IL-4 and IL-10 reduced these effects of monocytes. We conclude that monocytic cells enhance the motility and invasion of tumour cells. These effects can be regulated by inhibitory cytokines of monocytes.