Protein in cerebrospinal fluid (CSF) was measured by a modified biuret procedure and in two automated instruments, the Du Pont aca and the Kodak Ektachem. The latter's dry-slide reagent was also evaluated for precision, linearity, and effect of potential interferents. In vitro, ampicillin and vancomycin increase the apparent value for CSF protein as measured with the Ektachem slides. We excluded patients with disorders of the central nervous system, and we estimated the central 95% percentile reference intervals for CSF protein for each of the three methods. We found no age or gender dependence of values. By the biuret procedure, the reference interval is 140 to 620 mg/L.