Histidine rich peptide (HRP) released from bovine high-molecular-weight kininogen by the action of plasma kallikrein is measured by a simple method employing bromocresol green (BCG). The addition of HRP to a BCG solution in a 0.1 M citrate buffer, pH 4.2 results in a decrease of dye level by a precipitation of dye level by a precipitation of dye-HRP complex. The reduced dye is linear for HRP up to 120 nmoles. Mole ratio of dye-HRP complex is approximately twenty.