In the past, the life-span of B cells in rodents has been determined by a variety of methods, leading to conflicting results. Among the various techniques employed, labeling of dividing cells with the thymidine analog 5-bromo-2′-deoxyuridine (BrdU) has turned out to be a versatile and reliable procedure. Labeling of the cells can be easily performed in vivo by feeding BrdU in the drinking water for extended periods of time or by an ip injection of BrdU for short-term labeling experiments. Using the protocol described, it is possible to combine flow cytometric detection of incorporated BrdU simultaneously with fluorescence staining of various cell surface markers.