Murine serum amyloid P component (SAP) is an acute-phase protein that is increased 2-10-fold in concentration following appropriate inflammatory or infectious stimuli. Previous studies of the acute-phase SAP response have employed quantitative immunoelectrophoresis or radioimmunoassay to measure SAP concentration. A rate nephelometric procedure has been developed which measures SAP concentration rapidly and with equivalent or greater precision than the previously applied techniques. This simple method will facilitate experimental and clinical studies of the acute-phase response.