study the effect of all trans retinoic acid (ATRA) and arsenic trioxide (As 2O 3) on tissue factor (TF) expression and procoagulant activity (PCA) of acute promyelocytic leukemia (APL) cells in vivo and in vitro Methods PCA from freshly isolated APL blasts from APL patients treated with ATRA or As 2O 3 was detected using a one stage clotting assay TF antigen was detected by ELISA and TF mRNA by RT PCR The maturation sensitive (NB4) or resistant subclones (NB4 R1) of the promyelocytic NB4 cell line, as well as U937 cells infected with pMSCV PML RARa treated with or without ATRA or As 2O 3, were also examined Results Both ATRA and As 2O 3 can down regulate the TF antigen, its mRNA transcription and membrane PCA of APL cells in vivo and in vitro, in a time dependent manner The TF antigen level in PML RARa+ U937 cells was significantly higher than that in U937 cells infected with retrovirus vector Both ATRA and As 2O 3 can also down regulate the TF antigen in U937 cells transfected with or without PML RARa Conclusion Tissue factor expression and PCA in APL cells may be down regulated by ATRA and As 2O 3 By down regulating TF expression, As 2O 3 might also be used to improve the DIC related hemorrhage in APL Our data indicate that elevated TF antigen in PML RARa+ U937 may be related to the fusion protein PML RARa The down regulating effect of ATRA and As 2O 3 on TF expression in U937 cells might not involve this fusion protein study the effect of all trans retinoic acid (ATRA) and arsenic trioxide (As 2 O 3) on tissue factor (TF) expression and procoagulant activity (PCA) of acute promyocytic leukemia (APL) cells in vivo and in vitro Methods PCA from freshly isolated APL blasts from APL patients treated with ATRA or As 2 O 3 was detected using a one stage clotting assay TF antigen was detected by ELISA and TF mRNA by RT PCR The maturation sensitive (NB4) or resistant subclones (NB4 R1) of the promyelocytic NB4 cell line, as well as U937 cells infected with pMSCV PML RARa treated with or without ATRA or As 2 O 3, were also examined Results Both ATRA and As 2 O 3 can down regulate the TF antigen, its mRNA transcription and membrane PCA of APL cells in vivo and in vitro, in a time-dependent manner in The TF antigen level in PML RARa + U937 cells was significantly higher than that in U937 cells infected with the retrovirus vector Both ATRA and As 2 O 3 can also down regulate the TF antig en in U937 cells transfected with or without PML RARa Conclusion Tissue factor expression and PCA in APL cells may be down regulated by ATRA and As 2 O 3 By down regulating TF expression, As 2 O 3 may also be used to improve the DIC related hemorrhage in APL Our data indicate that elevated TF antigen in PML RARa + U937 may be related to fusion protein PML RARa The down regulating effect of ATRA and As 2 O 3 on TF expression in U937 cells might not involve this fusion protein