Violacein is the main pigment produced by Chromobacterium violaceum, a saprophytic gram-negative bacillus. Violacein is formed by the condensation of two modified tryptophan molecules and has potential anti-neoplastic effects. The purpose of this pilot study was to investigate the in vitro activity of violacein in human uveal melanoma cell lines. Human uveal melanoma cell lines 92.1 and OCM-1 were incubated with five different concentrations of violacein (10−5–10−9 M), and the total cellular protein content was measured by means of the sulphorhodamine B assay. Dose–response curves were obtained and the concentration inhibiting cell growth by 50% (IC50) together with the concentration inhibiting the net cell growth by 50% (GI50) were calculated for both cell lines. Violacein IC50 and GI50 concentrations to cell line 92.1 were 2.78×10−6 M and 1.69×10−6 M, respectively. The IC50 and GI50 concentrations to cell line OCM-1 were 3.69×10−6 M and 2.12×10−6 M, respectively. Previous studies using the same methodology have revealed violacein to have a GI50 in the range (3–6)×10−8 M for MOLT-4 leukaemia, NCI-H460 large cell lung cancer and KM12 colon cancer cell lines. Violacein displayed borderline cytotoxic activity in human uveal melanoma cell lines 92.1 and OCM-1, as measured by the sulphorhodamine B assay, and further studies are necessary to define its suitability as a potential therapeutic agent for metastatic uveal melanoma.