Abstract: An apparatus (10) for detecting a particle (12) labelled with a fluorescent marker is disclosed. The apparatus (10) has a flow cell (16) being adapted to contain a fluid (14) in which the particle (12) is suspended. A light source (28) is operatively coupled to the flow cell (16) and arranged for emitting a stimulating light (28) which is effective in optically exciting the fluorescent marker (12) for emitting a fluorescent light (30). The apparatus (10) also includes a spatial filter (50) across an optical path between the particle (12) and a time gated detector (32) operatively coupled to the flow cell (16) for detecting the fluorescent light (30).