The occurrence of Marangoni and natural convection has been experimentally investigated during lysozyme and cytochrome c' protein crystal growth (PCG) using a vapor diffusion technique in a hanging droplet and a PCG apparatus. To detect the fluid motion, the photochromic dye activation and particle tracking velocimetry (PTV) techniques were used. In all the cases studied, the dye traces recorded by a CCD video camera did not show fluid movement with velocities greater than about 0.2 mum/s anywhere in the solution, even around the growing protein crystals. The PTV technique with a velocity resolution well below 0.2 mum/s also showed no detectable displacements of the tracer particles. These results indicate that an aqueous protein solution surface is highly resistant to Marangoni convection under typical PCG conditions. (C) 2003 Elsevier B.V. All rights reserved.