Polypeptides capable of forming antigen binding structures specific for Rhesus D antigens include the sequences indicated in the FIGS. 1a to 16b. The obtained polypeptides, being Fab fragments, MAY be used directly as an active ingredient in pharmaceutical and diagnostic compositions. The Fab and their DNA sequences can also be used for the preparation of complete recombinant Anti-Rhesus D antibodies. Useful in pharmaceutical and diagnostic compositions.