A procedure to filter the ICARUS digitized data is presented. The final multi-kiloton detector will provide raw data at a rate of the order of 100 Gbytes/s and therefore a crucial element of the acquisition chain will be the on-line data reduction system. A filter has been developed to process the signals and to tag the meaningful data. The hit detection efficiency has been evaluated on a sample of signals obtained from cosmic rays events in the 3 t ICARUS prototype. Its robustness has been tested also on simulated events with different signal to noise ratio. The hit finder hardware implementation is also presented.