IW32 cells were treated with camptothecin (0.1 μm), VM-26 (0.05 μm), or sodium butyrate (2.5 mm) for 48 hr, cells were harvested, and total cell lysates prepared as described in Materials and Methods. PTPase activity was assayed by usingp-Npp as a substrate according to Dunphy et al.(28). Vanadate-sensitive PTPase activity was determined by subtracting the activity in the presence of sodium vanadate (50 μm) from that obtained in the absence of the drug, whereas okadaic acid-sensitive PTPase activity was determined by subtracting the activity in the presence of okadaic acid (0.01 μm) from that in the absence of the drug. Results are mean ± standard deviation of three experiments, each performed in triplicate.