Results obtained by flow cytofluorometry are expressed in 'arbitrary' units, which have a relative significance only. A method is described to make this unit to represent a number of fluorochrome molecules per cell, i.e. to give it an absolute value. The method makes use of an antibody labeled with a fluorochrome (FITC) as well as a radioactive marker (125I). From the amount of radioactivity bound per cell on the one hand and the fluorescence measured by flow cytofluorometry on the other, the amount of FITC per unit of fluorescence follows by straight-forward calculation. The method described is rather simple to perform and is quite reliable. Some of its restrictions are discussed.