Quantitative data provided by flow cytometers are as yet not fully exploited due to the lack of standardization. However, fluorescence standardization systems are now available which allow the measurement of antigen density on a routine basis and the present review focuses on the interest of such quantitative techniques for the monitoring of haemopoietic malignancies. Antigen quantitation: (i) permits a more objective characterization of positivity, especially in the case of weakly expressed antigens; (ii) facilitates the analysis of complex populations, since certain antigens are expressed at different levels on different cell subsets; (iii) provides new data contributing to a more precise definition of cell differentiation; (iv) is of value to ascertain malignancy from the detection of aberrant antigen densities on putative neoplastic cells; (v) provides additional parameters suitable for the evaluation of residual disease and for the monitoring of immunological therapeutic regimens; (vi) contributes to prognosis. Quantitation of antigen densities should therefore be included in the routine study of haemopoietic malignancies.