Abstract: Particles in a liquid medium are detected by directing a source of laser light through a container containing a liquid sample with a concentration of microscopic particles in suspension. An optical detector located off the optical axis receives scattered light from the particles and provides a signal with a dc component and a varying component. The DC component is removed to provide a filtered signal representing the varying component. Movement is produced in the sample liquid, e.g., by convective stirring, to extend the band of frequencies of the varying component towards higher frequencies. The apparatus can be used for growth curve detection of biological samples and provides increased sensitivity.