In the half century since the first use of automated analyzers, manual techniques, especially microscopic examination of a stained blood film, have complemented analyzer results to provide a comprehensive hematology report on a patient's blood sample. Over the years, as the capabilities and performance of automated analyzers have improved, the respective roles of the automated analyzer and the complementary procedures have changed. Manual action (most commonly smear review) following automated analyzer results is usually triggered by determining whether the results trigger one of a series of criteria for review of results. There is little uniformity among different laboratories on criteria for action. Recognizing the long-standing need for generally accepted guidelines ("rules") which could be applied to criteria for review of CBC and differential results from automated hematology analyzers, Dr. Berend Houwen invited 20 experts to a meeting in the Spring of 2002 to discuss the issues and determine the most appropriate criteria. At this meeting, 83 rules were developed by consensus agreement. These rules were then tested in 15 laboratories on a total of 13,298 blood samples. After a detailed analysis of the data, the rules were refined and consolidated to produce 41 rules that are presented here. They include rules for first-time samples as well as delta rules for repeat samples within 72 hours from a patient. It is hoped that these rules will be useful to a large number of hematology laboratories worldwide. To facilitate validating these rules in individual laboratories before implementation in routine operation for patient samples, a suggested protocol is attached to this paper.