Abstract: A flow stream processor particularly adapted for use in blood cell processing, and having discriminator circuits for selecting a particular group of cells from a large population, thereby permitting digital processing of data relative to the cells to be carried on only with respect to the particular group. Additionally the system includes circuitry for automatically adjusting flow stream velocity in accordance with an error signal developed proportionally to the phase difference between signals received from the first and last detectors along the flow stream resulting from transmit of the same cell.