FLC OF A B757 DSNDED IN RESPONSE TO A TCASII RA WHILE LEVEL AT A DSCNT INTERMEDIATE ALT. ARTCC RADAR CTLR RPTED NO CONFLICTING TARGETS.

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I WAS THE CAPT AND MY FO WAS FLYING THIS LEG. WE WERE DSNDED ON COURSE FROM FL370 TO FL330 APCHING LAR; ON THE RAMMS ARR INTO DEN; AND ADVISED OF TFC AT OUR 12 O'CLOCK AT FL310; THEN DIRECTED BY ATC TO TURN R 20 DEGS AND EXPECT LOWER WHEN CLR OF TFC. SHORTLY THEREAFTER; WE RECEIVED A RED TCASII CONFLICT ALERT; INDICATING TFC AT OUR 12 O'CLOCK POS AT FL330! THIS WAS FOLLOWED BY A DSND COMMAND ON TCASII WHICH WAS IMMEDIATELY FOLLOWED BY FO. THE ALERT SUBSIDED AT APPROX FL323 AND WE LEVELED OFF AT FL320. ATC WAS IMMEDIATELY ADVISED WE WERE IN A TCASII DSCNT; AND AGAIN ADVISED WHEN WE LEVELED OFF AT FL320. NEITHER MYSELF NOR THE FO SAW ANY SIGN OF TFC. WX WAS CLR AND VISIBILITY WELL OVER 50 MI AT OUR ALT AND BELOW. THE TFC WE HAD BEEN ADVISED OF EARLIER WAS VISIBLE ON THE TCASII INDICATING FL310 SOON AFTER WE LEVELED OFF AT FL320. ATC RPTED NO TFC AT OUR ALT IN OUR AREA AND CLRED US TO CONTINUE DSCNT TO FL270 WHEN CLR OF THE FL310 TFC; AND THE REMAINDER OF THE ARR WAS WITHOUT INCIDENT. I HAVE HAD NUMEROUS TFC ALERTS SINCE THE INSTALLATION OF THIS EQUIP ON THE B757 AND B767 ACFT; BUT THIS WAS THE FIRST TCASII CONFLICT COMMAND I HAVE SEEN IN THE ACFT. THE MANEUVER WAS PERFORMED FLAWLESSLY BY MY FO; AND NO PROBS OCCURRED AS A RESULT. I AM CONCERNED ABOUT A 'DOWN' COMMAND WHEN 12 O'CLOCK TFC WAS AT FL310!