Cell kinetic measurements are currently being investigated to determine if they are useful in the clinical management of patients with non-Hodgkin's lymphomas (NHLs). Although the tritiated thymidine labeling index (TLI) is the standard method of S-phase measurement, it is difficult to perform. The authors describe a slide-based immunofluorescence labeling index (LI) method that uses 5-bromo-2-deoxyuridine (BrdUrd) as the pulsing medium and a monoclonal antibody (BU-1) to BrdUrd. The BrdUrd LI was performed on 217 NHLs and compared with routine histologic results. The authors found a median BrdUrd LI of 0.9% for low-grade NHLs; 7.5% for intermediate-grade; 10.4% for high-grade; and 2.2% for T-cell NHLs. This method provides a rapid, reliable S-phase measurement that can be easily performed in the clinical laboratory. It should replace the TLI and allow wider application of S-phase measurements in the NHL.