This famous North Side theater was formerly a distinguished in-the-round venue whose artistic director, George Keithley, brought audiences up close to famous stars (such as Sandy Dennis in ''A Streetcar Named Desire''), created a show, ''Status Quo Vadis,'' that went on to Broadway and premiered Tennessee Williams' ''The Two Person Play'' in 1971. It is now operated by owner Douglas Bragan, who has added a studio theater and a third, even more intimate space to the complex. (Note: the extreme side seats in the mainstage theater can make you feel very removed from the action.) The theater has been a cooperative outlet for City Lit Theatre Company's celebrated ''Jeeves'' series and a transfer house for such successes as Apple Tree Theatre's ''Three Hotels,'' Black Ensemble Theater's ''Doo Wop Shoo Bop'' and a revival of the Gershwins' ''Strike Up the Band.'' Other long-running productions hosted by the Ivanhoe are the comedies ''Late Night Catechism,'' ''Hellcab'' and various Free Associates improv shows. Excellent seats, fair acoustics and efficient air conditioning.