WASHINGTON (AP) — Interest rates on short-term Treasury bills were mixed in Tuesday's auction with rates on three-month bills falling to the lowest level since late April while rates on six-month bills were unchanged.

The Treasury Department auctioned $28 billion in three-month bills at a discount rate of 0.025 percent, down from 0.030 percent last week. Another $24 billion in six-month bills was auctioned at a discount rate of 0.050 percent, unchanged from last week.

The three-month rate was the lowest since these bills averaged 0.020 percent on April 28.

The discount rates reflect that the bills sell for less than face value. For a $10,000 bill, the three-month price was $9,999.37 while a six-month bill sold for $9,997.47. That would equal an annualized rate of 0.025 percent for the three-month bills and 0.051 percent for the six-month bills.

The Treasury bill auction, normally held on Monday, was delayed until Tuesday this week because of the Labor Day holiday.

Separately, the Federal Reserve said Tuesday that the average yield for one-year Treasury bills, a popular index for making changes in adjustable rate mortgages, was 0.11 percent last week, unchanged from the previous week.