5-Bromo-2'-deoxyuridne (BrdU) and 3H-thymidine label mitotically active cells, but they do not adequately mark the progeny of dividing cells for long term study. An alternative method is to label cells using the replication-defective CXL retroviral vector, which carries the lacZ gene encoding beta-galactosidase; however, the ability of the CXL retroviral vector to pulse-label mitotically active cells selectively is not known. Cultures of proliferating muscle cells were simultaneously incubated with the CXL retrovirus and BrdU (10 microM) for 2 hr. After removing the retrovirus containing medium, the cells were maintained for an additional 24 hr in vitro before they were stained to detect beta-galactosidase and BrdU simultaneously. More than 95% of beta-galactosidase positive cells were also BrdU positive suggesting that the majority of beta-galactosidase positive cells were in the S-phase of the cell cycle at the time of CXL retroviral administration. Therefore, the CXL retroviral vector is an appropriate pulse marker for dividing cells, and it is useful when it is desirable to know the fate of the progeny of a particular cell following a mitotic event.