We obtained enzyme data on normal individuals in conjunction with a large interlaboratory enzyme survey. For the 12 largest peer groups using unique methods, we found a simple relationship between the upper reference limits and the laboratories' results obtained from human-enzyme-supplemented survey serum. A conversion to a common base made possible the merging of data on the normal individuals and interconversion of results by diverse methods. We determined the upper reference limits for serum lactate dehydrogenase, alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, and creatine kinase for approximately 8000 adult men and women believed to be in good health. Using a technique described here, we believe that the results can be transformed to user-specific units, and that the large data base can be applied to the many diverse enzyme methods in current use. With these data, enzyme survey participants will be able to estimate appropriate reference intervals for their particular method.