This study was done to investigate the behavior of concrete under three-dimensional stresses with two objects in mind. The first object was to determine the effect of the lateral stresses on the longitudinal strength of a concrete specimen with empty voids. The other was to determine the effect of the interstitial pressure on the strength of the mnterial. To achieve these two objects cylindrical concrete specimens were subjected to triazial compressive stresses with two lateral stresses constant and equal and the specimens brought to failure under axial compression. Some specimens were tested with empty voids and others, with a liquid under pressure in the pores. To create the first of these conditions a rubber tubing was used on the specimen to protect it from the liquid pressure. In the second case the liquid was free to penetrate the specimen voids. The relative difference, between the two, in the strength of the material gives the effect of liquid under pressure in the pores. The influence of lateral stresses on the axial strength of a concrete member has a direct application in reinforced concrete columns. Also, this influence would help us to understand the behavior of concrete under various stresses and be useful in formulating a theory of failure. The case of liquid under pressure in the voids has a direct application to structures that are subject to hydraulic pressure. This is called the effect of pore-water pressure, or uplift, and it is important to understand its effect on stability for the design of any hydraulic structure. It is suggested by the latest theories that this effect can be determined only by experimental means. There is at present no definite idea about either the exact potential area of surface or the part of it on which the pore pressure acts.