We describe a refined version of the particle scattering apparatus due to van de Ven (Langmuir 1994, 10, 3046) for measuring the dynamic interactions between colloidal particles in a flow field. Polystyrene latex particles (diameter 5.1 mu m) were studied, stabilized by beta-casein and alpha(s1)-casein, at pH 7 and ionic strengths of 0.1 and 0.01 mol dm(-3). The beta-casein system exhibited stronger interparticle repulsion and less tendency for particle flocculation at the lower ionic strength. At ionic strength 0.1 mol dm-3 the alpha(s1)-casein system exhibited stronger interparticle repulsion than the beta-casein system. This behavior is consistent with other measurements on the properties of casein-coated particles.