The man and a male suspect were involved in a verbal altercation on a westbound No. 79 bus just before 9:30 a.m. at 79th Street and the Red Line station in Chatham, police said.
The male suspect pulled out a sharp object, and cut the man, police said.
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The man suffered a laceration to the lower back, and was taken to Advocate Christ Medical Center in Oak Lawn in serious condition.
No one else was injured, and no one is in custody later Wednesday.
Detectives are investigating.
CTA officials confirmed the "disturbance."
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