Flow cytometry, as the name suggests, is the analysis of cells (which carry one or more fluorescent labels) moving in a fluid flow (1 ,2 ). This technique has become widely used because of the enormous increase in the number and range of specificities of antibodies to cell determinants. Monoclonal and polyclonal antibodies to both murine and human antigens, indeed antibodies that have a fluorochrome covalently attached (and which have already been tested in various assay systems), are readily commercially available.