In this paper we describe an indirect fluorescence double staining procedure for the simultaneous detection of IdUrd and CldUrd in the same cell nucleus. Two commercially available antibodies were selected for this purpose. A rat antiBrdUrd monoclonal antibody from Seralab was found to bind specifically to CldUrd and BrdUrd. A mouse monoclonal anti-BrdUrd antibody from Becton Dickinson used in a 1:2 dilution binds to all halogenated deoxyuridines but, when the cells were extensively washed with Tris buffer with a high salt concentration, almost no binding to CldUrd was observed. An immunofluorescence procedure was developed, based on these primary antibodies, raised in different species (rat and mouse), in combination with highly purified second antibodies: FITC conjugated goat antirat and TexasRed conjugated goat antimouse.Key terms: Immunochemistry, double staining procedure, IdUrd, CldUrd, flow cytometry, cell kineticsThe study of experimental and human tumors has been facilitated by the recent introduction of immunocytochemical procedures for the detection of halogenated deoxyuridines incorporated into cellular DNA (1,2,(4)(5)(6)(7)(8)(9)(10)(11)13,15,17,18). These methods are rapid and easy to perform, and data from a single sample can provide several cell kinetic parameters (cf. cell cycle time, duration of S-phase, and the potential doubling time) (3). However, not all relevant cell cycle kinetic parameters can be studied by this technique. Recruitment of cells can only be studied when a combination of DNA labels is used that can be detected separately. Detection of a second label in the same cell population requires the development of a n immunocytochemical double staining procedure using a pair of monoclonal antibodies with high specificity for different halogenated deoxyuridines and with low cross-reactivity.Most commercially available antibodies show crossreactivity between the two most frequently used halogenated deoxyuridines: bromo-and iododeoxyuridine (BrdUrd and IdUrd). Monoclonal antibodies with different specificities were reported by Vanderlaan et al. (16): Br-3, which should only recognize BrdUrd, and IU-4, which recognizes both BrdUrd and IdUrd. Shibui et al. (14) recently developed a n immunocytochemical staining procedure using these antibodies for the detection of IdUrd and BrdUrd given to the same cell population. A major disadvantage of this procedure is that the IU-4 antibody recognizes both labels IdUrd and BrdUrd. Moreover, since this reaction is based on enzyme reactions, it is not suitable for cells in suspension, hence is unsuitable for flow cytometric application. We recently studied several commercially available monoclonal antibodies for their specificity for binding to bromo-, iodo-, and chlorodeoxyuridine. Although nearly all monoclonal antibodies examined reacted with all the different halogenated deoxyuridines, we were able to select a pair of antibodies that, under the chosen experimental conditions, showed a large difference in binding to IdUrd and CldUrd. Using these com...