The present invention relates to a flow cytometer (10) for in vitro assaying of human or animal whole blood and to an investigation method using the flow cytometer. Enhanced detection properties are achieved by it relative to the prior art cytometers of the same kind. Here, automated beam positioning is also solved. To these ends, collection of light scattered by the cellular components of human or animal whole blood and its transmission to suitable optical sensing elements take place by a coupling member with a particular end construction, in particular through one or more optical fiber bundles. Preparation of a blood sample for the assay, that is, mixing up said human or animal whole blood with appropriate reagents is performed by a hydro-pneumatical unit (12) of a particular design. Moreover, the actual assaying takes place in a flow cell (22) of a particular construction, which assists to improve the signal-to-noise ratio of the present flow cytometer.