SPRAY-DROP assay results do not always meet the Poisson-distribut ion test proposed by Williamson and Taylor1, even though they may be valid and the sole product of expensive measurements. Williamson and Taylor have described an appropriate method for interpreting the spray-drop results obtained by the technique of Williams and Backus2,3 for estimating the concentration of particles in a suspension. In this technique the unknown suspension is mixed with a suspension of polystyrene latex spheres in known concentration. The mixture is sprayed as a fine mist, and droplets are collected on specimen supports for examination by electron microscopy. For each droplet, the number of particles and the number of latex spheres observed are recorded. Counted for many droplets, these numbers constitute the raw data from which the concentration of particles in the unknown suspension is deduced.