Twelve patients undergoing IL-2 and flavone acetic acid (FAA) combination immunotherapy for advanced melanoma were studied throughout treatment for the induction of measurable levels of bioactive TNF, GM-CSF and IL-6 in their serum. This was to assess the extent of secondary cytokine induction in these patients and the possible role of such cytokines in both the toxic and therapeutic responses. The nature of the treatment schedule enabled these cytokines to be measured in response to FAA alone, FAA/IL-2 and FAA alone following IL-2/FAA activation of target cells. A small rise in the serum levels of these cytokines was seen on the initial course of FAA/IL-2 but this was minor compared to the marked elevation in levels 2-8 h following the initiation of the third course of FAA given with or without IL-2 and at a time point which coincided with maximum toxicity in those patients who experienced it. These results show that FAA alone can induce cytokine release from primed target cells. This may be associated with the therapeutic effect and/or toxicity of the agent.