POTENTIAL CONFLICT. 1 ACFT WITH TCASII; THE OTHER NOT.

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IN LEVEL CRUISE FLT AT 12500 FT; VISUALLY CONTACTED TFC AT OUR 10 O'CLOCK POS; APPROX 2 MI ON COLLISION COURSE. IMMEDIATELY INITIATED DSNDING R TURN. OTHER ACFT MAINTAINED ITS HDG BUT ALSO INITIATED DSCNT. AT THE TIME OF OCCURRENCE; WE WERE RECEIVING TFC ADVISORIES FROM ATC. THROUGH POST OCCURRENCE DISCUSSION ON THE FREQ IT BECAME APPARENT THAT THE OTHER ACFT WAS RESPONDING TO COMMANDS FROM ITS TCASII. WE WERE NOT TCASII EQUIPPED. (SEVERAL SECONDS AFTER INITIATING OUR EVASIVE MANEUVER ATC ALERTED US TO THE TFC INCLUDING THAT THE OTHER ACFT WAS CLBING.) ONE NOTEWORTHY OBSERVATION IS THAT IN THE UNIQUE SIT WHERE A CONFLICT EXISTS BTWN A TCASII EQUIPPED AND A NON- TCASII EQUIPPED ACFT WHERE THE NON-TCASII EQUIPPED PLT FIRST MAKES VISUAL CONTACT AND INITIATES EVASION THE TCASII SYS CAN ACTUALLY WORK AGAINST COLLISION AVOIDANCE. HERE I ASSUME; THE TCASII COMMAND WAS BASED ON OUR ACFT MAINTAINING LEVEL FLT AND WAS SLOW IN RECOGNIZING OUR DSCNT; ALSO; THE CTLR CLAIMED THE OTHER ACFT WAS IN A CLB; PRESUMABLY JUST BEFORE IT STARTED ITS DSCNT. MUCH GREATER SEPARATION WOULD HAVE RESULTED IF THE OTHER ACFT HAD NOT RESPONDED TO TCASII AND STAYED ITS COURSE AND CLB.