Abstract: Methods for detecting the presence or amount of Interleukin-12 (IL-12), particularly human IL-12, or antibodies that neutralize the activity of human IL-12 in a human biological sample, involve incubating the biological sample with an NK cell line or progeny thereof. A correlation is determined between the presence or amount of IL-12 (or anti-IL-12 antibodies) in the sample and the amount of gamma interferon (or a precursor activation factor or cytokine) secreted by the NK cells. For measuring antibodies, the sample is pre-incubated with IL-12. Levels of a cytokine or activation factor, such as IFN-?, secreted by the NK cells when in the presence of IL-12 are then compared with a control level of the cytokine or activation factor secreted by the NK cells or progeny thereof in the presence of a known amount of IL-12 and without the biological sample. The presence of IL-12 or neutralizing anti-IL12 antibodies in the sample is indicated by a variation, e.g.