Two modes of continuous isoelectric focusing are described. The development of a natural pH gradient, consisting of a mixture of three buffer solutions, and the focusing behavior of human serum albumin is investigated. The advantages of isoelectric focusing in an artificial pH gradient of three buffer solutions are demonstrated on the purification of alpha-amylase from an E. coli protein extract. Furthermore the principle of field step electrophoresis is presented. The most important factors influencing the efficiency: (i) residence time, (ii) conductivity of the sample and (iii) sample zone width, are discussed. The use of a larger sized device to allow simultaneous multiple injections of the sample demonstrates the feasibility of scaling-up field step electrophoresis. This approach permits a throughput of about 20 mL sample solution per minute.