Two conventions for preparing dilutions are used in clinical laboratories. The first convention defines an "a:b" dilution as "a" volumes of solution A plus "b" volumes of solution B. The second convention defines an "a:b" dilution as "a" volumes of solution A diluted into a final volume of "b". Use of the incorrect dilution convention could affect the sensitivity and results of an assay. This could have serious consequences for a patient's diagnosis. Ideally, all solution concentrations should be expressed as molarity, molality, or normality, but this is frequently undesirable in commercial clinical assay instructions for proprietary reasons. To avoid confusion, it is critical that a statement be included at the beginning of the experimental portion of each protocol explicitly describing how dilutions are to be performed. The American Chemical Society and educators could establish and teach an unambiguous standardized dilution convention. This would ultimately eliminate the problem.