Objective    To determine the effect of pneumolysin (Ply) on proliferation and apoptosis in human acute monocytic leukemia THP-1 cells.     Methods     After exposure to Ply at 0.05

0.1

0.5

1

2.5

or 5 μg/ml for 24

48 or 72 h

the proliferation in THP-1 cells was measured by MTT assay. Morphological changes in the treated and untreated THP-1 cells were observed by Wright’s staining. The apoptotic rate of THP-1 cells was examined by

AnnexinV-FITC/PI-stain. DNA fragmentation was examined by sepharose electrophoresis. The expression of MAPK1/3 was investigated by immunocytochemical staining and Western blot analysis.     Results     Ply resulted in apparent inhibition on proliferation of THP-1 cells in a time- and dose-dependent manner

with an IC

50

（24 h） of 1.31  μg/ml. Typical morphological features of apoptosis

such as nuclear fragmentation and apoptosis bodies

were observed in THP-1 cells after Ply treatment at 0.05 and 0.1  μg/ml for 12 h by Wright’s staining. After Ply treatment at 0.05 and 0.1  μg/ml for 12 h

the apoptotic rate of THP-1 cells was 11.83% and 48.45%

respectively (P0.05). A typical ladder pattern was identified in DNA electrophoresis. The expression of MAPK1/3 was reduced (P0.05) after THP-1 cells were treated with Ply.     Conclusion      Ply induces apoptosis in human THP-1 cells though MAPK1/3 signal pathway.