An attachment for the Linear Transport System of the Gilford 2000 recording spectrophotometer is described which allows in situ continuous scanning of protein zones subjected to isoelectric focusing in sucrose gradients. Microgram quantities of protein samples can be analyzed in a short time (1.5 to 3 hr) in a quartz microcolumn. Resolution of separated components is excellent. Application of the method to the isoelectric separation of the commercially available proteins ovalbumin, beta lactoglobulin, myoglobin, Bowman-Birk soybean protease inhibitor, and rabbit gamma globulins is reported. The method provides for scanning at different wavelengths for differentiation of colored proteins, or proteins lacking tryptophan. The described procedure offers an opportunity for the studying of the kinetics of isoelectrofocusing in sucrose gradients.