Understanding how immune cells such as macrophages interact with cancer cells is of increasing interest, as cancer treatments move towards combining both targeted- and immuno- therapies in new treatment regimes. This protocol is using THP-1 cells, a human leukemia monocytic cell line that can be differentiated into macrophages. This allows studying the effects of the macrophage secretome on cancer cells (on e.g. growth, drug response or gene expression) in co-cultures without direct cell contact interactions. This is an important aspect as it removes the presence of any phagocytic aspect to changes in the cancer cell number and behaviour. The in vitro THP-1 monocyte differentiation into polarized macrophages was used to study the effects of both M1 and M2 type populations of macrophages on melanoma cells (Smith et al., 2014; Tsuchiya et al., 1980). M1 type macrophages are classically thought to be tumour suppressing as opposed to M2 type macrophages, which are thought to possess tissue repairing and tumour growth promoting activities.