For carboxyfluorescein (CFSE) proliferation studies, MB114 cells and Sll cells were washed twice in PBS (five minutes, 10006g) and resuspended in a solution of PBS and 2% fetal calf serum (buffer B) at a concentration of 1610 6 cells per mL. An equal volume of 4 mM CFSE in buffer B was added to the cell suspension (2 mM final concentration) and pipet mixed. After three minutes the labeling reaction was quenched with an equal volume of fetal calf serum for 30 seconds, and buffer B was added for a total volume of 50 mL. Cells were centrifuged (five minutes, 10006g), resuspended in buffer B, and aliquots collected for day 0 analysis. Remaining, labeled cells were resuspended in complete media and cultured. Labeled MB114 cells were infected at an MOI = 5 PFU/ cell. Infected cells were harvested as described at day six postinfection and analyzed by flow cytometry at six and 12 days post-infection. Stained and unstained S11 cells were analyzed at the same time as a positive control.