Coadministration of Herceptin-coated tumor cells and IL-12 results in increased circulating levels of IFN-γ in vivo. Herceptin-coated CT-26^HER2/neu murine tumor cells were injected intraperitoneally concurrently with 1 μg muIL-12 (Her/CT-26^HER2 + IL-12). Control groups included mice (n = 10) that received normal huIgG-treated tumor cells only (IgG/CT-26^HER2), Herceptin-coated tumor cells only (Her/CT-26^HER2), or huIgG-treated tumor cells plus IL-12 (IgG/CT-26^HER2 + IL-12). Serum was harvested from each mouse at 6, 12, and 24 hours and analyzed by ELISA for levels of IFN-γ and TNF-α. Use of the parental CT-26 cell line in this assay did not result in cytokine production by any condition examined (not shown).