A new type of cubical triaxial apparatus was used for an investigation of the influence of the intermediate principle stress on the drained stress-strain and strength characteristics of sand. For the same value of the minor principle stress and increasing value of the intermediate principle stress, both dense and loose specimens showed increasing slope of the stress-strain curve, decreasing strain-to-failure, and increasing rate of dilation. The strength of the sand as represented by the friction angle increased from triaxial compression to a maximum before a slight decrease at triaxial extension. Observations regarding the applicability of the normality condition of classical plasticity theory to sand were made. Expressions were derived for the variations of the friction angle in the immediate vicinity of the conditions of triaxial compression and extension, and the test results were shown to fit these expressions.