A new class of fluorescent dye which upon excitation at 488 nm turns red is shown to be probe-suitable for using in flow cytometry alone or in conjunction with fluorescein derivatives. 7-dimethylamino 3-(p-formylstyryl) 1,4 benzoxazin 2-one is suitable for rendering microorganisms, such as Plasmodium merozoites and cells detectable by flow cytometry, allowing a dual fluorescence analysis when the cells are labelled with suitable fluoresceinylated ligands such as fluorescein labeled neoglycoproteins or antibodies. The synthesis of the new benzoxazinone derivatives is described: p-[beta-(7-dimethylamino 1,4 benzoxazin 2-one 3-yl)-vinyl]-phenylpropenoic acid can be easily activated as a hydroxysuccinimide derivative and linked to amino groups of polypeptides. Hydrophilic polypeptides such as poly-L-lysine or glycosylated polymers combined with this new fluorescent dye are shown to be helpful in analyzing cell surface receptors, in dual fluorescence flow cytometry analysis, using a single excitation wavelength and two sets of compounds labeled with the new benzoxazinone derivative and with fluorescein isothiocyanate, respectively. The new benzoxazinone derivative has a high molar absorbance, a good quantum yield fluorescence when it is bound to hydrophilic polypeptides and its fluorescence intensity is not dependent on pH in the physiological pH range.