For networks, putting up a promising new show to knock off a rival's established hit has long been an iffy business. This season, ABC is running its critically acclaimed new series, Murder One, in the Thursday slot opposite NBC's reigning ratings champ, ER. So far, ER, starring Anthony Edwards, is still No.1; the crime show, No.51. But in the past, some feisty newcomers have dislodged entrenched hits: Bill Cosby squelched Magnum, P.I. in 1984-85.

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