AN ACR MD80 HAD LTSS WITH AN UNK ACFT. THE RPTRS HAD BEEN GIVEN CLB CLRNC FROM ZAU AND WERE CLBING INTO ZOB'S AIRSPACE.

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IN CRUISE AT FL250 AND WITH ZAU FREQ 127.62; ORD CTLR ISSUED A CLB TO FL330; DIRECT SVM; AND INSTRUCTIONS TO CONTACT ZOB ON 135.72. CLRNC WAS READ BACK TO CTR BY CAPT PRIOR TO SWITCHING FREQS. BOTH OF US ACKNOWLEDGED AND CONFIRMED THAT FL330 WAS SET AND ARMED. I BEGAN A VERT SPD CLB OF ABOUT 1200 FPM AND COMMANDED .75 MACH FOR THE AUTOTHROTTLES. CAPT WAS SIMULTANEOUSLY CHKING IN WITH ZOB AND CALLED DEPARTING FL250 FOR FL330 WHICH WAS ROGERED BY THE CTLR. SECONDS LATER; A YELLOW TA APPEARED CLOSE IN AT +1000 FT. AS I DISENGAGED THE AUTOPLT; THE TA BECAME AN RA 'REDUCE CLB' FOLLOWED BY 'DSND; DSND.' CAPT INFORMED CTR WE WERE RESPONDING TO AN RA AND DSNDING. THE TFC PASSED R TO L INSIDE 1 MI AND; AS OBSERVED ON TCASII; 200 FT ABOVE. CAPT OBSERVED THE TFC IN THIN CIRRUS AT OUR 9- 10 O'CLOCK AND VERY CLOSE IN GOING AWAY. CTLR HAD US REMAIN AT FL250 AND ASKED WHO HAD GIVEN US FL330. WE TOLD HIM THAT ZAU 127.62 HAD. WE INFORMED CTLR OF OUR INTENTIONS TO FILE A NMAC AND CONTINUED TO DEST WITHOUT FURTHER DIFFICULTIES. COMMENT: AN INOP TCASII SHOULD PERHAPS BE A NO-GO ITEM PER THE MEL -- IT SURELY SAVED US THIS TIME.