Determination of the number of mononuclear cells that secrete (i.e. produce and release) interferon gamma (IFNG), found in a specified sample of spleen, the highly vascular lymphoid organ which serves to store blood, disintegrate old blood cells, filter foreign substances from the blood, and produce lymphocytes. Ifng is a member of the type II interferon family which is a soluble cytokine with antiviral, immunoregulatory and anti-tumor properties and functions as a potent activator of macrophages.