The determination of proteins with tetrasodium 2-(3,6-disulfo-8-hydroxynaphthylazo)-1,8-dihydroxynaphthalene- 3,6-disulfonate (Beryllon II) by Rayleigh light scattering (RLS) was studied. The weak RLS of the Beryllon II–bovine serum albumin (BSA) complex can be greatly enhanced by the addition of Al³⁺ in the pH range 5.6–7.2; there was a maximum RLS platform at 400–420 nm. Based on the reaction between Beryllon II, Al³⁺ and proteins, a new method for the determination of proteins was developed. This method is very sensitive [0.20–41.42 μg ml⁻¹ for BSA and 0.18–48.15 μg ml⁻¹ for human serum albumen (HSA)], rapid (<2 min), simple (one step) and tolerant towards most interfering substances. The effects of different surfactants were also examined. Four samples of protein in human serum were determined; the maximum relative error was no more than 5% and the recovery was 96–105%.