AN ACR B727 CAPT COMPLAINS ABOUT THE TAXI PROC AT ORD.

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FOLLOWING LNDG AND EXIT RWY 9R; INSTRUCTED BY TWR TO SWITCH TO 121.9 FOR TAXI. GND CTL CLRED US (ACR X) TO 'TURN L ON 'A' AND PROCEED TO S ENTRANCE.' WHILE TAXIING ON 'A'; IN CLOCKWISE FLOW; ABOUT A PLANE LENGTH BEHIND AN ACR Y FOKKER; OBSERVED AN ACR Z MD-80 EXITING THE ACR Z ALLEY. THE ACR Z PLANE WAS ABOUT OUR 2 O'CLOCK POS; 3 OR 4 PLANE LENGTHS FROM CTRLINE TXWY A; AT INITIAL OBSERVATION. AS WE CONTINUED OUR TAXI ON 'A'; ACR Z CONTINUED EXIT OF ALLEY; ANGLING ITS APCH TO TXWY A; SO AS TO CREATE A COLLISION COURSE WITH US. WE QUERIED GND CTL AS TO INTENTIONS OF ACR Z PLANE AND ASKED IF THEY WERE IN COMS. FOLLOWING SEVERAL SECONDS OF SILENCE; GND CTL REPLIED THAT THEY 'DIDN'T KNOW WHAT THE ACR Z PLANE WAS DOING;' AT WHICH POINT I STOPPED TO AVOID POSSIBLE COLLISION. THE ACR Z PLANE CONTINUED ITS IRREGULAR ENTRY ONTO TXWY A AND SUCCESSFULLY 'CUT US OFF.' THE ACR Z CAPT (IN CLR VIEW FROM OUR COCKPIT POS) NEVER ACKNOWLEDGED OUR PRESENCE VIA RADIO OR VISUALLY. GND CTL APOLOGIZED FOR THE MISHAP. WE CONTINUED OUR TAXI BEHIND THE ACR Z PLANE ON 'A' TO THE ACR X S ENTRANCE EXITING THERE. ALTHOUGH NO OFFICIAL EXPLANATION GIVEN; I SUSPECT THAT THE ACR Z PLANE WAS NOT ON FREQ 121.9 AT TIME OF ENTRY ONTO TXWY A. THAT SOLUTION IS OBVIOUS. HOWEVER; EVEN IF ON A DIFFERENT FREQ; A PIC SHOULD NOT IGNORE A POTENTIALLY HAZARDOUS SIT AND HOPE THAT THE OTHER PIC ACTS RESPONSIBLY. BOTH ACFT SHOULD HAVE STOPPED.