State Street Global Advisors launched five bond exchange-traded funds on the American Stock Exchange on Wednesday.

Three of the new State Street SPDR ETFs track Treasury bond indices of various maturities, one tracks an inflation-protected securities index, and another tracks an investment-grade bond index.

SSGA, a unit of State Street ( STT), was the first to market with a stock ETF 14 years ago when it launched the Standard & Poor's Depositary Receipt ( SPY), also called the SPDR. But the company has been relatively slow out of the starting gate with bond ETFs; its first five offerings follow on the heels of two other providers: Barclay Global Investors, a unit of Barclays PLC ( BCS ) and Vanguard.

Still, the fixed-income asset class is relatively untapped by ETFs, which are essentially index-oriented funds that trade on an exchange. With the five launched Wednesday, the total number available in the U.S. has more than quadrupled over the past five months to 25. At the end of 2006, there were only six bond ETFs, all of them BGI's iShares. Since then, BGI has almost tripled its offerings. Vanguard got into the game in April with four bond offerings.

"We've been working on a fixed-income product for years," says Gary MacDonald, State Street's director of marketing. "But the regulatory environment was hard." He says it can take a long time to get approval the Securities and Exchange Commission to sign off on a new ETF, though the process has been getting better.

The SPDR Lehman 1-3 Month T-Bill ( BIL) tracks the Lehman Brothers 1-3 Month U.S. Treasury Bill Index, which is up 1.24% so far this year and has returned an annualized 3.35% over the last three years. The ETF's expense ratio is

The SPDR Lehman Intermediate Term Treasury ( ITE) tracks Lehman's index of Treasury notes with a maturity greater than or equal to one year and less than 10 years. This benchmark is up 1.52% to far this year and has returned an annualized 2.38% over the past three years. The expense ratio is 0.1345%.

The SPDR Lehman Long Term Treasury ( TLO) tracks the index of the same name, which measures the performance U.S. Treasury bonds with a maturity of 10 years or more. So far this year, this index advanced 1.05%, with a three-year annualized return of 3.87%. The expense ratio is 0.1345%

The SPDR Lehman Aggregate Bond's ( LAG) benchmark index of U.S. dollar-denominated investment-grade bonds is up 1.5% this year, and 3.31% annualized for the past three years. The expense ratio is 0.1845%

The SPDR Barclays Capital TIPS ( IPE) tracks the performance of Barclays' index of Treasury bonds whose principal is adjusted to the consumer price index. The benchmark is up 2.43% this year, and it has a 2.95% annualized return for the past three years. The expense ratio is 0.1845%

The fees on State Street's bond ETFs are higher than Vanguard's which carry expense ratios of 0.11%, but lower than those of BGI's, which range from 0.15% to 0.5%.