This paper describes an instrument, the Laboratory Sprayer, which has been designed to evaluate the compatibility of pesticide tank mixes by a method significantly more representative of field application than the commonly used ball jar test. The Laboratory Sprayer has been designed to incorporate as many features of the full-scale equipment as possible, including constant agitation, circulation through a filter, and spraying through a nozzle to simulate the dynamics of a field application. In contrast to the static ball jar test, this sprayer has been found to show excellent correlation with field conditions.