Four different protein precipitants, namely trichloroacetic acid, sulphosalicylic acid, benzethonium chloride and benzalkonium chloride, were used to estimate the total protein concentration in cerebrospinal fluid and urine by nephelometry. Protein determinations for 50 cerebrospinal fluid samples and 100 urine samples were compared with values obtained by a trichloroacetic acid-Ponceau S spectrophotometric method. All methods were correlated well, but total protein results using acid precipitants were usually lower, while results obtained using benzethonium chloride were generally higher, than results obtained by the trichloroacetic acid-Ponceau S method. Anomalous results were obtained for some urine samples with benzethonium chloride, but not with benzalkonium chloride. Assays using benzalkonium chloride as precipitating reagent showed good precision and closest agreement with the trichloroacetic acid-Ponceau S dye-binding method. The use of benzalkonium chloride as precipitant is recommended for automated cerebrospinal fluid and urine protein estimations by nephelometry.