The automatic determination of hormone levels in serum by column chromatograhoic methods using a 25 channel peristaltic pump, allows the simultaneous chromatography of 25 samples. The different volumes which are necessary for the successive elution steps are set by the tubing size of the pump and by a preset switch clock. The eluates from the 25 columns are collected simultaneously in fractionated volumes which are also determined in the same electronic clock device. The assays so far adapted to this principle are reviewed: Serum thyroxine (competitive protein binding assay), T3-uptake test (dextran gel filtration), triiodothyronine (radioimmunoassay) and cortisol (competitive protein binding assay or radioimmunoassay). The main advantages of this procedure are increased frequency, improved specificity and standardization at lower costs for these methods. The principle of simultaneous column chromatography appears to be most suited for assays, which require various extraction steps prior to the determination of the hormone, since it allows extraction, specific protein binding and bound/free separation on only one column.