The emission anisotropy of selected fluorescent probes which interact with cells and their membranes is a sensitive parameter for studying the structural changes associated with different functional states. Such measurements can now be made on individual living cells at rates of up to 103 per second and the cells separated on the basis of the anisotropy function alone or combined with other physical signals using a multiparameter automated computer-controlled cell separator (MACCS). Thus, selection can be on the basis of simple or complex algorithms reflecting the size, macromolecular content, and rotational mobility of cellular components or liganded reporter molecules. Cells isolated in this manner are sterile, viable, and can be used for outgrowth or biochemical studies related to dynamic changes occurring during differentiation or malignant transformation.