Abstract: A method and apparatus for determining the presence of particles in a fluorescent dispersion which exceed either a predetermined fluorescence intensity or size are disclosed. The method and apparatus involve the irradiation of the dispersion with excitation light, the examination of the fluorescent light entering an optical fiber from a portion of the excited region small enough to permit only a low probability that more than one particle of interest is present in the volume during a predetermined period of time, and the measurement of the emitted fluorescence entering the fiber, substantially free of excitation light, to the extent that the fluorescence differs in intensity from that of the dispersion in the absence of the particle.