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Monolithic active pixel sensors constitute a viable alternative to hybrid pixel sensors and charge coupled devices for the next generation of vertex detectors. Possible applications will strongly depend on a successful implementation of on-chip hit recognition and sparsification schemes. The task is tough, first because of very small signal amplitudes (∼ mV), which are of the same order of magnitude as natural dispersions in the transistor threshold voltages, secondly because of the limitation to use only one type of transistor over the sensitive area. This paper presents a 30×128 pixel prototype chip, featuring fast, column parallel signal processing. The pixel concept combines in-pixel amplification with double sampling operation. The pixel output is a differential current signal proportional to the difference between the reference level and the charge collected. The readout of the pixel is two-phase, matching discrimination circuitry implemented at the end of each column. Low-noise discriminators feature autozero functionality. The details of the chip design are presented. Difficulties, encountered in the first attempt to address on-line hit recognition, are reported. Performances of the pixel and discriminator blocks, determined in separate measurements, are discussed. An important part of this paper consists of results of first tests performed with soft X-rays from a 55Fe source.