[en] A description is given of the design and performance of an instrument which measures particle size and radioactivity of individual microparticles after uptake of a γ-emitting radioactive tracer. A flow system allows the activated particles to be measured quickly and individually, giving statistically significant frequency distributions in a short time. The purpose of the flow device is to measure some property of particles of a given type, provided this property is correlated with the uptake of the tracer. The instrument sizes particles in the 5--150-μm-diam range, and detects γ rays of E>30 keV