FO OF ACR MLG ACFT DEVIATED FROM ASSIGNED ALT IN RESPONSE TO A TCASII RA.

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EVENT OCCURRED ON OUR ATL TO SAT LEG WHILE FLYING THE MARCS 1 ARR FROM INDUSTRY. WE HAD LEVELED AT 12000 FT TO CROSS MARCS AND I WAS FLYING THE ACFT (MLG) VIA THE AUTOPLT. I HAD BEGUN TO SLOW TO 250 AS WE TURNED TO CRISS. APCH CTL CALLED VFR TFC AT 11.5 AT 12-1 O'CLOCK AND THEN ASKED IF WE WANTED TO LAND ON 12R OR 21 -- WINDS 210 AT 10. THE CAPT REPLIED 'STANDBY' AND TOLD ME WE SHOULD LOOK FOR THE TFC. WE WERE BOTH LOOKING OUTSIDE FOR A NOSE-ON (SMA) WHEN THE TCASII ALERTED 'TFC; TFC.' I LOOKED AT THE TCASII SYMBOLOGY AND SAW THE YELLOW CIRCLE AT APPROX 2 MI ON THE NOSE. I WAS QUITE CONCERNED (142 PEOPLE ON BOARD) AND CONTINUED TO LOOK OUTSIDE. IN A MATTER OF SECONDS THE TCASII WENT TO A TA 'CLB; CLB' WITH A GREEN ARC AT APPROX 2000 FPM. IN ACCORDANCE WITH COMPANY POLICY AND MY BELIEF THAT INACTION WOULD BE WRONG; I IMMEDIATELY DISENGAGED THE AUTOPLT AND HAND-FLEW THE RA TO 2000 FPM. AS SOON AS TCASII ALERTED 'CLR OF CONFLICT;' I SLOWLY NOSED OVER TO BEGIN A DSCNT. I TRIED TO MAKE THIS MANEUVER AS SMOOTH AS POSSIBLE AND I'M NOT COMPLETELY SURE HOW HIGH WE GOT. I BELIEVE I STARTED THE NOSE OVER AT APPROX 13000 FT. APCH ASKED WHY WE DID THIS; SINCE THEY CALLED OUT THE TFC. QUITE SIMPLY; I DIDN'T SEE THE TFC AND I WOULDN'T BET 142 PAX PLUS 6 CREW THAT THE VFR TFC WAS AT 11.5. I WOULD DO THE SAME AGAIN. SAT APCH FILED AN ALTDEV. ADDITIONAL CONSIDERATIONS: WILL VFR TFC AT 500 FT SEPARATION ALWAYS GIVE A TCASII RA? IF SO; SHOULD PLTS NOT IN VISUAL CONTACT WITH THE VFR TFC COMPLY WITH THE RA? IT WOULD SEEM TO ME THAT 500 FT SEPARATION SIMPLY IS NOT COMFORTABLE AND APCH SHOULD VECTOR IFR CARRIERS AWAY FROM KNOWN VFR TFC IN RADIO CONTACT IF THE SEPARATION WILL BE 500 FT. SAN ANTONIO APCH WAS UPSET THAT WE DEVIATED; BUT IN MY MIND THERE WAS NO OTHER POSSIBLE ACTION UNLESS I HAD A VISUAL.