Flow cytometry and monoclonal antibodies have combined to provide an extraordinarily powerful technology for the analysis of blood cell populations and their phenotype and function. Flow cytometry has facilitated the development and characterisation of monoclonal antibodies and their target antigens, wile monoclonal antibodies have stimulated the development of flow cytometric instruments and techniques. In this presentation, the synergies are illustrated by reference to the HLDA Workshops, large international collaborations that analysed hundreds of antibodies in a "blind" format that brought confidence and order to what had been a chaotic field. As a result, monoclonal antibodies against leucocyte antigens have provided powerful research and diagnostic reagents, as well as the basis for therapeutic approaches in cancer and other diseases involving the immune response. Although flow cytometric analysis of cells in suspension was the first to contribute to and benefit from this body of work, it has been extended increasingly to analysis of solid tissues using microscopy.