The in meso method for growing crystals of membrane proteins uses a spontaneously forming lipidic cubic mesophase. The detergent-solubilized protein is dispersed with lipid, typically monoolein, and in so doing the cubic phase self-assembles. A precipitant is added to trigger crystal nucleation and growth. The commercial screen solution series are convenient for use in crystallization trials. The aim of this study was to determine which of the Hampton Screen and Screen 2 series of solutions are compatible with the in meso method. These screens contain components any of which could destroy the cubic phase. X-ray diffraction was used for phase identification and for microstructure characterization. The study was done at 4 degrees C and at 20 degrees C. Two types of sample preparations were examined. One used an excess of half-strength screen solution (Prep. 1). The other used a limiting quantity of undiluted screen solution (Prep. 2). At 20 degrees C, over 90% of the screen solutions produced the cubic phase with Prep. 1. This figure dropped to 50% with Prep. 2. In contrast, 50 to 60% of the screens were cubic phase compatible at 4 degrees C under Prep. 1 conditions. The figure fell to 25% with Prep. 2. The mode of action of the diverse screen components are explained on the basis of the phase properties of the monoolein/water system.