The amount of light scattered by particles in a suspension depends on the concentration and geometry of the particles, and on the optical properties of the suspension. in the paper a new method is presented to extract particle concentration and a characteristic particle sire from turbidimeter readings. The method requires a turbidimeter capable of measuring the forward (12 degrees) and sideways (90 degrees) scattered light simultaneously. The turbidimeter must be calibrated with several samples with known particle concentration and characteristic size. Results are presented for the calibration of filter aids, an industrial pigment and yeast. An example is given of its intended use: the on-line analysis of filtrate turbidity. The method is of course limited to the range of particle concentration (1-200 ppm) and characteristic particle size (1-100 mu m) used in the calibration. A second limitation arises from the fact that other parameters influencing scattering (optical properties, particle shape and porosity, width of the size distribution) are not considered. The anomalous behaviour of baker＇s yeast can be explained in this way.