The device serves for the qualitative and quantitative detection of particles in a fluid and includes a light source (1), an optical sensor (2) and a sample carrier (4) which is arranged therebetween and which is for receiving fluid to be examined. The sample carrier is (4) movable relative at least to the sensor (2) and is connectable via a fluid inlet (9) to a conduit (11) for feeding the fluid, and via a fluid outlet (10) to a conduit (12) for the discharge of fluid. The sample carrier (4) is exchangeably arranged in a receiver of the device, so that this sample carrier can be replaced by another one in a rapid and simple manner when the sample carrier (4) is contaminated.