Improved method for detecting and/or counting the appearance of hematopoietic progenitor cells which comprises the steps of mixing a blood sample with a reagent capable of detecting immature cells without employing any immunological techniques, obtaining cell information about the thus treated sample using a particle analyzer and constructing a cell distribution profile, delineating a portion of the profile as a zone in which at least one subclass of hematopoietic progenitor cells appear, and counting the cells within the zone. The method is capable of detecting the appearance of hematopoietic progenitor cells in a simple and positive manner.