B190 FLC INITIATES EVASIVE MANEUVER WHEN OBSERVING OPPOSITE DIRECTION; SAME ALT; DC9.

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AT APPROX XA45 LCL; WE WERE OPERATING AS A PART 121 SCHEDULED PAX FLT WITH A DEST OF MKE. IN CRUISE FLT AT 10000 FT; WE NOTICED A TARGET ON THE TCASII AT OUR ALT AND IN OUR 11 O'CLOCK POS AT 10 MI. I ACQUIRED THE APCHING ACFT VISUALLY AT APPROX 5 MI IN FRONT OF US. I POINTED THE ACFT OUT TO THE CAPT AND I STARTED TO SEE THE APCHING ACFT BANK L; THUS TURNING TOWARDS US. THE ACFT WAS NOW APPROX 2 MI IN FRONT OF US; STILL AT OUR ALT; TURNING TOWARD US AND CLOSING AT A HIGH RATE OF SPD. THE CAPT AND I BOTH AGREED EVASIVE ACTION WAS NECESSARY AND JUST AS WE GOT A TA FROM THE TCASII; THE CAPT STARTED A RAPID DSCNT TO 9000 FT MSL. THE OTHER ACFT APPEARED TO STAY AT THEIR ALT OF 10000 FT AS IT PASSED OVER US. THIS INCIDENT OCCURRED AT A POINT AT WHICH HDOFS ARE NORMALLY MADE BTWN ZAU AND MKE APCH CTL. THE OTHER ACFT WAS NOT ON OUR FREQ AND WE ARE FAIRLY SURE THAT THEY WERE IN CONTACT WITH MKE APCH. I BELIEVE THAT THE FACT THAT WE WERE BEING CTLED BY ZAU ON THE 'BORDER' OF MKE'S AIRSPACE WAS A SIGNIFICANT FACTOR IN THE CONFLICT. SOUND JUDGEMENT AND GOOD CREW COORD PROVED TO BE INVALUABLE IN THIS EVENT.