Results of a 1994 study at the National Aggregates Association/National Ready Mix Concrete Association's Joint Research Laboratory show that interpreting aggregate ASR predictive test results can be a challenge. The study's results may confirm producers' claims that service records may be a more reliable indicator of acceptability than accelerated laboratory tests. ASTM C 1260, Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method), provides a result in about two weeks. ASTM C 1260 measures the expansion of a mortar bar containing aggregates from a specific source as it undergoes severe exposure conditions attempting to accelerate any potential expansion that might occur in field concrete. C 1260 has been used to reject aggregates with good service records. Some engineers, concerned with the poor correlation between aggregates with good service records and results of C 1260, are turning to the more recently published ASTM C 1293, Standard Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction. Also known as the "concrete prism test", this predictive test measures a cylinder's expansion over a one-year period less aggressively than C 1260. The Canadian Standards Association (CSA) found that this test produces results that better correlate with the field performance and that ASTM C 1260 results should be used only as a quick screening test and not as a basis for rejecting aggregates. keywords: ASR, test, ASTM C 1260, ASTM C 1293, mortar bar, prism