Abstract The flow-injection system is based on the use of Folin-Ciocalteus reagent, with 1,4-dithio-D,L-threitol to accelerate the formation of the coloured complex. The calibration graph for bovine serum albumin is linear between 0.01 and 0.1 mg ml −1; the detection limit is 0.005 mg ml −1 albumin and up to 0.5 mg ml −1 can be determined. The injection frequency is 20 h −1. The results obtained by the flow-injection method and by the manual Lowry procedure show excellent correlation. Egg lysozyme, alkaline phosphatase and β-lactoglobulin are also tested.