The effects of the Echinacea on superoxide anion production by the promyelocytic cell line, HL-60, were evaluated by culturing the cells with a 55% ethanol extract of the aerial parts of Echinacea purpurea. HL-60 cells cultured with the 55% ethanol extract of E. purpurea alone at 28 µg/ml produced statistically significant levels of superoxide anion compared to vehicle-treated controls (p < 0.05). These data suggest that the 55% ethanol extract of E. purpurea contains immunomodulatory compounds capable of stimulating superoxide anion production by HL-60 cells; and, as such, may have implications for therapeutic use.