The McLean, VA.-based company said it would start enforcing the new standards after Sept. 1, 2007.
Shockwaves have been rippling through financial markets as more signs emerge that relaxed lending standards during the housing boom of recent years are leading to escalating defaults and rising losses for lenders and owners of securities backed by such loans.
For its part, Freddie Mac said Tuesday that it would stop buying those mortgages that have "a high likelihood of excessive payment shock and possible foreclosure." Instead, the company plans to buy only subprime adjustable-rate mortgages, and securities backed by such loans, that have been qualified at the fully indexed and fully amortizing rate.
The firm said its new requirements cover mortgages known as 2/28 and 3/27 hybrid ARMs, which currently make up about three-quarters of the subprime market.