A new method of protein crystallization, the floating-drop vapour-diffusion technique, has been developed. The method combines the traditional sitting-drop vapour-diffusion technique and a novel two-liquid system. A crystallization drop composed of a mixture of sample and reagent floats on an insoluble and very dense liquid. Protein crystals are grown by vapour diffusion at the interface of the two liquids. The method makes it possible to remove the crystals easily without causing mechanical damage. This approach also significantly reduces the time and cost compared with the hanging-drop technique, which is presently the most popular method for protein crystallization.