The effect of human C‐reactive protein (CRP) on macrophage function was studied in an assay of superoxide anion (O₂⁻) production. Peritoneal exudate macrophages (PEM) of guinea pigs exposed in vitro to various doses of CRP for 72 hr resulted in the development of O₂⁻ production dose‐dependently, measured by increases in superoxide dismutase‐inhibitable nitro blue tetrazolium reduction. The O₂⁻‐producing activity of PEM cultured without CRP, used as a control, decreased markedly in proportion to incubation time. The O₂⁻ production by PEM exposed to CRP for 18 hr when control PEM were still high in O₂⁻ production, was decreased by larger doses of CRP, while PEM cultured without CRP for 72 hr, when O₂⁻ production by control PEM was very low, followed by incubation with CRP for another 18 hr, produced O₂⁻ CRP‐dose‐dependently as in the case of that observed after 72‐hr incubation with CRP. These results indicate that CRP is capable of activating macrophages and acts on macrophage function as a modulator. CRP possesses migration inhibitory factor (MIF)‐like activity (as reported in the preceding paper) and also macrophage‐activating factor (MAF)‐like activity, indicating that CRP may play a functional role at the site of inflammation and tissue damage by accumulating and activating macrophages.