MANY specimens for electron microscopical examination are in the form of suspensions in salt solution. These are commonly dried on the specimen support film, and the unwanted salt crystals are removed by washing, with resultant uncertainties due to loss of material during the process, which losses may well be selective. The salts are sometimes first removed from the suspensions by dialysis; but this may lead to changes in aggregation, or to loss by precipitation before the material can be finally laid down.