A murine hybridoma cell line that produces a monoclonal antibody to human Hageman factor (HF, factor XII) is described. The antibody (P 5–2– 1) consists of mouse IgG2b heavy chains and lambda light chains, selectively neutralizes HF procoagulant activity, and prevents the proteolytic cleavage of HF during contact activation in plasma. When HF is exposed to P 5–2–1 before the absorption of HF to kaolin, HF procoagulant activity is markedly inhibited. In contrast, P 5–2–1 does not interfere with HF activity after the adsorption of HF to kaolin. P 5–2–1 does not inactivate the prekallikrein–activating activity of 28,000–mol wt HF fragments (HFf). P 5–2–1 binds exclusively to the 40,000mol wt portion of a heavy chain of HF and inhibits the adsorption of HF to negatively charged surfaces. P 5–2–1 immobilized on Sepharose can be used to deplete HF from normal human plasma. This immunoaffinity-depleted plasma is indistinguishable from congenital HF- deficient plasma and can be used as the substrate for HF procoagulant activity assay.