Standard & Poor's Corp. cut its ratings on another $5.19 billion of U.S. collateralized debt obligations Tuesday, reflecting credit deterioration and recent ratings cuts on residential mortgage-backed securities amid continuing housing weakness.

The agency trimmed ratings on 19 tranches from seven cash-flow and hybrid CDOs and removed 13 of them from review for possible downgradings. Of the others, six face a significant likelihood of further rating reductions. S&P also affirmed one rating and removed the security from review for possible downgrading.

Four of the seven transactions are mezzanine structured-finance CDOs collateralized by residential mortgage-backed securities and by other asset-backed securities. The other three transactions are high-grade structured finance CDOs, which were collateralized mostly by AAA- through A-rated tranches of residential mortgage-backed securities.

So far, S&P has cut ratings on 3,899 tranches from 879 U.S. cash-flow, hybrid, and synthetic CDO transactions as a result of stress in the U.S. residential mortgage market and credit deterioration of residential mortgage-backed securities. In addition, 1,237 ratings from 445 transactions are on review for possible downgradings.

S&P has downgraded $457.24 billion of CDOs, and its ratings on $28.42 billion of securities face a high likelihood of downgrading.

S&P has been cutting ratings on billions of dollars of alternative-A and subprime residential mortgage-backed securities deals in recent weeks after revising the amount of losses expected. The changes occurred as delinquencies climbed and home prices fell.

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