We have systematically investigated the capacity of highly purified human peripheral CD8+ T cells to produce interleukin (IL)-4 and interferon (IFN)-γ when triggered by different stimuli. CD8+ T cells were isolated from peripheral blood by positive selection to > 99% purity and stimulated with one of three different stimuli: phytohaemagglutinin (PHA) and IL-2, phorbol myristate acetate (PMA) and ionomycin, and plate-bound anti CD3 and PMA. On their own, ionomycin and IL-2 failed to stimulate significant CD8+ T cell proliferation while PHA, plate-bound anti-CD3 and PMA induced weak proliferation. A combination of PHA and IL-2, PMA and ionomycin, or plate-bound anti-CD3 and PMA all induced vigorous CD8+ T cell proliferation. IFN-γ was produced following all three stimuli, but was greatest from cells cultured with PMA and ionomycin. However, IL-4 secretion was only detected in cell cultures stimulated with PMA and ionomycin. These results indicate that, with sufficient stimulation, human CD8+ T cells have the potential to produce Th2 as well as Th1 cytokines.