Dick Guttman, a spokesman for the 67-year-old actor, says Keach was in stable condition. He didn't say why Keach was hospitalized or provide further details.
Keach had been starring as Richard Nixon in the Center Theater Group's production of "Frost/Nixon" at the Ahmanson Theatre in Culver City.
Center Theater Group spokesman Ken Werther says Keach was to be replaced by his understudy, Bob Ari, for Tuesday's performance.
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Associated Press Writer Jill Lawless in London contributed to this report.
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