We describe a sensitive and reproducible microassay model using human peripheral blood lymphocytes (PBL) for discrimination between the cytotoxic and immunosuppressive effects of food colorants such as amaranth and tartrazine. The cytotoxic effects of a wide range of concentrations of these substances were studied on human PBL by the colorimetric in vitro cytotoxicity assays, neutral red uptake (NR) and thiazolyl blue tetrazolium bromide (MTT). The immunotoxic properties of these 2 substances were determined by a [3H]-thymidine DNA incorporation assay on phytohemagglutinin stimulated or non-stimulated lymphocytes, as well as by a Cr51 release Natural Killer assays. The results showed clear immunosuppressive effects from the 2 substances tested, although the concentrations chosen for this study proved to be non-cytotoxic by NR and MTT cytotoxic endpoints.