Some homebuyers will have a harder time finding loans in the spring because a major government-sponsored mortgage buyer is tightening its standards.

The Federal Home Loan Mortgage Corp., known as Freddie Mac, said that after March 31 it will stop buying two types of mortgages: adjustable-rate loans with less than a 10 percent down payment and low-documentation loans.The rule changes come as a growing number of lenders are tightening credit terms on a variety of loans, creating what some borrowers say is a credit crunch.

In addition, after Jan. 2, Freddie Mac is now more closely scrutinizing its purchases of loans originated by mortgage brokers and other third parties working for lenders.

The change on low-down-payment, adjustable-rate mortgages will pinch first-time homebuyers the most, consumer advocates and economists say. The restriction on low-documentation mortgages affects mostly buyers moving up to more expensive homes.

Many renters, especially in high-cost areas on the East and West coasts, can raise only a 5 percent down payment to purchase their first home.

"Housing prices have gone up so much that ARMs (adjustable-rate mortgages) have been about the only tool available to make it possible for some people to buy a home," said Peggy Miller of the Consumer Federation of America.