Reduced expression of the myristoylated alanine-rich C kinase substrate (MARCKS) has been described in various cell lines after oncogenic or chemical transformation, leading to the question of whether this protein may be involved in cell proliferation. Here we compare the expression of MARCKS in human tumor-derived choroidal melanoma cells (OCM-1) and in primary cultures of normal choroidal melanocytes. We found an important down-regulation of the protein in the melanoma cell line. Stable transfection of these cells with the cDNA coding for MARCKS led to the selection of several clones expressing variable levels of the protein. Proliferation experiments performed with four of these clones revealed that cell growth was reduced by 35-40% when compared with control cells. Upon serum starvation, cell proliferation was almost abolished when the expression level of MARCKS was high, whereas it was only partially reduced in the controls. MARCKS overexpression induced a higher percentage of cells in the G0-G1 phase of the cell cycle upon serum starvation, as well as the inhibition of colony formation in soft agar. Finally, the expression of the CDK inhibitor p27 was increased in the cells presenting a high level of MARCKS protein. Altogether, these data suggest that the expression of this protein kinase C substrate affects the proliferation and partially reverts the transformed phenotype of the OCM-1 cells.