Electrophoretic separation of non-specific esterases and esterproteases from kidney, lung, and liver have been carried out in polyacrylamide gels. By use of zone electrophoresis, isoelectric focusing, and 2-dimensional electrophoresis it was found that most of the esterprotease bands had the same localization in the gels as non-specific esterase bands. A number of esterase bands showed no activity towards the esterprotease substrates and a single kidney band possessed esterprotease activity only. Isozymes of the ES-6 and ES-9 zones showed sex dependent esterprotease reactions. In sections esterase activity was located to all parts of the proximal tubule. In male kidneys, esterprotease activity was present in the 2nd segment of the convoluted tubule which is not connected to glomeruli and in the descending part of the proximal tubule. In female kidney only the descending part of the proximal tubule showed esterprotease activity.