α₂-Macroglobulin (AMG) and C-reactive protein (CRP) levels in cerebrospinal fluid (CSF) of patients with bacterial and aseptic meningitis have been analyzed by a rate nephelometric method to determine if these acute phase proteins can aid in differentiation of bacterial from aseptic meningitis. The mean CSF concentrations of AMG and CRP were 15 and 3.5 times greater, respectively, in the bacterial compared to the aseptic meningitis group. Also, the range of AMG levels showed minimal overlap between the two groups. The elevated levels of the proteins persisted after CSF cultures became negative. Quantitation of specific acute phase proteins in CSF may assist the differentiation of bacterial from aseptic meningitis.