A discrepancy in current serologic typing and historical results can cause consternation in the transfusion service technologist finding the discrepancy. This program will include a discussion on an approach for investigating discrepant results seen between historical typing using human-source, polyspecific reagents or when using another reagent consisting of a different clone. A review of uncommon or rare phenotypes showing variable reactivity with different reagents will be provided. Finally, case studies will show how these serologic problems have resulted in the discovery of new or rare antigens