A new design of mixer for seed dressing is presented. The main objective in the design was to provide a uniform stress field without high shear spots or eddy zones. The approach was to adopt the concept of rotational cylinder viscometry and apply the analysis of shear stresses and shear rates to a vertical axis mixer. The resulting design consisted of a stationary outer chamber with an inner rotor. Flights mounted on the rotor, interleaved with baffles attached to the inside of the chamber wall to ensure that all the seed was circulated through the treatment zone. The clearances and speeds were carefully evaluated to make sure that maximum intensity of mixing was achieved without any danger of damage to the seed. The shear rate at the flight tip was evaluated as (41 \mathrm{~s}{ }^{1}). Power predictions were estimated on a basis of friction of grain on the moving surfaces and gave an overestimate of (23 %) but nevertheless proved to be useful in choosing a drive motor. Tests in the laboratory correlated well with those in the full-scale test machine on the basis of three criteria: the mean residence time of the seed in the mixer, the visual appearance of uniformity of the chemical treatment and the measurement of damage. The mean residence time varied from (22 \mathrm{~s}) at a rate of (29 \mathrm{t} / \mathrm{h}) to (88 \mathrm{~s}) for a rate of (5 \mathrm{t} / \mathrm{h}). The uniformity was judged to be as good as that achieved by other commercial mixers and the amount of damage caused to the seed was found to be statistically insignificant. The commercial model is a very compact machine compared to other seed dressers. It mixes grain thoroughly with a mean residence time of under (1 \mathrm{~min}) and causes no significant damage to the grain.