Determination of the Rh(D) binding epitope of anti-Rh(D) Fab/phage clones. The fine specificities of monoclonal Fab/phage clones were determined by their ability to agglutinate members of a panel of six Rh(D) variant RBCs. Shown are the five different agglutination patterns obtained from screening all of the 53 Fab/phage clones. The particular clones shown are identified by their unique heavy chain/light chain pairings using the nomenclature defined in Figs 1, 4, and 5. For E1/M3, reactivity with additional Rh(D) variant cells would be required to distinguish its specificity for epD3 versus epD9.3 Rh(D) epitope assignments are as per Lomas et al.6 Note that inclusion of the category IVb cell (not available in our previous study)22 permits the identification of a new epitope designated epDX (see text).