Melanoma inhibitory activity (MIA) is correlated with tumour progression and development of metastatic disease. Melanoma inhibitory activity, secreted by melanoma cells, is known to inhibit tumour cell attachment to the extracellular matrix enhancing their invasive potential. The regulatory pathways that lead to MIA expression have not yet been elucidated. It is well established that tumour cells and macrophages interact through soluble factors, preventing or enhancing tumour growth. The purpose of the present study was to determine whether soluble factor(s) derived from macrophages lead to the up-regulation of MIA production by human uveal melanoma cell lines (HUMCL) and whether MIA contributes to an increase in the invasive behaviour of HUMCL in vitro. Baseline MIA levels were measured by enzyme-linked immunosorbent assay in five HUMCL of known metastatic potential (92.1>SP6.5>OCM-1>MKT-BR>UW-1). Macrophage conditioned medium (MaCM) was placed on top of the HUMCL and MIA levels were measured at 6, 12, 24, and 36 h. The HUMCL were also seeded in a Matrigel chamber for 72 h and then cells invading the Matrigel were counted. The assay was repeated adding recombinant human MIA to the top layer of each well. All HUMCL expressed MIA at baseline (average of 31 ng/ml at 36 h). Following exposure to MaCM, MIA levels increased to an average of 45.2 ng/ml, with the 92.1 and SP6.5 cell lines expressing the highest MIA levels and UW-1 cell line expressing the lowest level. During the baseline invasion assay, the vast majority of cells (>95%) were found to adhere to the upper surface of the Matrigel. When MIA was added to the invasion chamber, no adhesion or invasion was observed. The results suggest, for the first time, that macrophages secrete a soluble factor(s) that may stimulate nearby melanoma cells to enhance their production of MIA in vitro. Furthermore, increased MIA production may, in turn, increase the invasive properties of the cells by modulating the attachment of HUMCL to the extracellular matrix.