Virus particles suspended in a drop of water tend to concentrate at or near its surface with the air. The concentrated, and probably more purified, particles may then be collected on a film-coated grid for negative staining and electron microscopy. This is a useful method, simpler than others (e.g. high-speed centrifugation, Lyphogel, or precipitation by (NH4)2SO4) which are used to process clinical specimens for diagnosis where virus particles may be too dilute in the original sample. It is shown, by freeze-fracturing for electron microscopy, that Orf virus particles do accumulate at and just below the surfaces of drops. Various physical effects which may cause the particles to accumulate are considered. Results from a computer model suggest that Brownian motion alone could be adequate to transport a useful quantity of the particles in the body of a 2-mm-diameter hemispherical drop to its surface if the particles do not clump and if they remain trapped at the surface when they reach it. In practice, transport by Brownian motion is likely to be augmented by swirling, convection and other effects within drops.