So, is your patient or donor a group O with missing isoagglutinins in the plasma or a weak subgroup of A? Is the Rh D typing record from another facility D negative but you are getting D positive? What is the difference between the reagents used in the past and what you are using now? What are some of the explanations for differences between serology and molecular test results? This session will explain how the blood grouping and typing reagents are manufactured (in the past and present) from human and animal source material, seeds (lectins) and murine and human derived monoclonal antibodies. Case studies will be used to illustrate different serologic and molecular results obtained when testing unusual samples with the various ABO and Rh D reagents, other Rh reagents, as well as MNS, Kell, Duffy and Kidd.