Multiangle dynamic light scattering (MDLS) and turbidimetry (T) were applied (both individually and combined) for determining the contamination by larger particles of two almost-uniform polystyrene (PS) latices. Latex I was synthesized in our laboratories, and it contained a main population diameter of 340 urn together with a small fraction of larger particles. This latex was used as the base material for producting an immunoassay kit. Latex 2 was obtained by a simple blend of two uniform PS standards. The proposed data treatment calculates the diameter and number fraction of the large particles contamination assuming that the PSDs are bimodal. The calculation involves minimizing the errors between the measurements and their theoretical predictions. When analyzed by combined MDLS-T, the contamination of Latex I involved number fraction 0.6% and particle diameter 865 run. The T average diameter is a function of the measurement wavelength, and the highest deviations of this average to all increasing, contamination by large particles were always observed at the higher wavelengths. The DLS average diameter is a function of the measurement angle. but ill this Case it is impossible to determine a priori the angle of observation that provides the largest deviation of this average diameter to all increasing contamination. (c) 2004 Elsevier Inc. All rights reserved.