Carboxylesterase activity in intact THP-1 cells is inhibited by chlorpyrifos oxon, paraoxon, or methyl paraoxon. THP-1 cells were incubated with chlorpyrifos oxon (A), paraoxon (B), or methyl paraoxon (C) for 24 h at the indicated concentrations. Following treatments, cells were collected, washed, and lysed. Carboxylesterase activity was measured in cell lysates and is expressed as % of control (mean ± SD, n = 3).