Standard & Poor's Corp. said Monday that it has downgraded $21.18 billion of collateralized debt obligations, citing deteriorating credit quality and rating cuts on subprime residential mortgage-backed securities.

It was one of the largest groups of CDOs downgraded by S&P in recent months. Ratings on hundreds of billions of dollars of alternative-A and subprime residential mortgage-backed deals have been cut as delinquencies continue to climb and housing prices keep falling.

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