A new technique has been developed for producing microspheres uniform in volume and fluorescence or radioactive tracer. Very uniform spheres with selected diameters are produced by introducing one liquid into the center of a moving stream of a second liquid (of differing surface tension) and breaking the core stream into uniform droplets by the action of a piezoelectric transducer. Fluorescent spheres of 10 &mgr; diam have been produced having a coefficient of variation in volume and fluorescence as low as 3.6%. This corresponds to a diameter variation of about 1.5%.