Minimizing or removing formation damage is a major objective in completion and stimulation operations. Formation damage is minimized by selection of "nondamaging" fluid systems and operations on the basis of petrographic analysis and field experience. A technique known as the capillary suction time (CST) test, adopted from the drilling-fluid industry, provides a quick and inexpensive method for qualitative selection or screening of the least damaging fluid system for a particular formation. The CST test is a fundamental filtration method for determining the electrolyte concentration that will produce the maximum inhibiting effect on a formation. The method uses an instrument to measure the time required for a liquid to travel a calibrated distance on a standard porous filter paper. The CST procedure currently is used very successfully to select completion fluids and to evaluate stimulation-fluid additives. This paper explains the procedure, discusses limitations of the test, and presents laboratory and field data to show the value of the technique.