ELECTROPHORESIS and ionophoresis on paper are being widely used, and a number of different forms of apparatus for carrying out this technique have already been described. The purpose of this communication is to give further details of an apparatus already briefly described1, which has been used for a considerable time with satisfactory results. While we do not claim that it is superior to other types of apparatus, it has the advantage of simplicity, results are reproducible, and the movement of a given substance varies linearly with time. The cross-section is shown in the accompanying diagram. The apparatus consists of a flat rectangular glass tray in which stands a closely fitting glass frame which carries the paper strip and rectangular ‘Perspex’ electrolyte boxes. The dimensions of the apparatus are not critical, but glass drip-trays from refrigerators are very convenient, the outside measurements being about 35 cm. × 20 cm. × 5 cm. The frame is constructed of two rectangles of glass rod, mounted one over the other, and separated from each other by about 1.5 cm. by means of sealed-on pieces of glass, or by pieces of cork at the corners. The frame stands on short legs to keep the paper away from the floor of the tray. Each electrode box has a hole drilled through the end about 1 cm. above the bottom. This enables electrolyte-levels in both boxes to be balanced prior to electrophoresis, and also permits old electrolyte to be run out and replaced by fresh, should this be desired.