This paper presents the results of Talwalkar-Parmelee plasticity measurements of a number of clays, and an interpretation of the results in the light of data on dye adsorption, water of plasticity, drying and firing shrinkage, and dry transverse strength. The clays studied are mainly from California, but tests of a few more widely-known clays from other sources are included for the purposes of correlation. Some data are given to show the effect of the addition of fine sand on the plasticity of a clay. The effect of variations in the water of plasticity is also considered. 1