Determining serum CA 19-9 using regular Centocor kit frequently requires numerous repeats when specimens contain highly elevated levels of CA 19-9. We found that the number of repeats could be reduced if the Centocor kit with an extended concentration range was used. Using an extended Centocor kit could also eliminate falsely low CA 19-9 results. However, when a Biomira RIA kit, based on a competitive binding principle, was used, falsely low CA 19-9 values could be completely avoided and, in most cases, only a single repeat was usually required to arrive at the true result. This is because the level of CA 19-9 in the specimen can be approximated from the radioactivity count. Therefore, when designing an immunoassay for tumor markers, one needs to take into consideration the concentration range of the tumor marker in the specimens if one would like to avoid repeats and falsely low values.