Some recent red-hot initial public offerings have the Securities and Exchange Commission concerned that Wall Street's underwriters may be tempted to revive some troubling tech bubble practices.

You can't help but be concerned by IPO valuations, Robert Khuzami, the SEC's enforcement chief, told an audience of Wall Street lawyers and compliance officers in New York on Tuesday.

A combination of first-day trading spikes -- such as a 109 percent advance by social networking company LinkedIn Corp last month -- and razor-thin yields on bonds and other securities, may create an environment where Wall Street investment banks can take advantage of investor demand.

It hasn't been that long ago that allocation practices were at the forefront of everyone's mind, and charges were brought against firms for Reg M and aftermarket violations for using their allocation process for creating demand in the aftermarket, Khuzami said at a luncheon hosted by the Securities Industry and Financial Markets Association.

Khuzami said the SEC will look to see whether or not these issues give rise to practices that we saw historically and that troubled us.

LinkedIn's shares on May 19 more than doubled in price on their first day of trade on the New York Stock Exchange, evoking memories of the frenzied dot-com bubble years. The advent of online brokerage accounts and a nation of day traders helped then start-ups like theglobe.com, VA Linux and MarketWatch.com rise by six- and seven-fold.

Many of these bubble babies burned through their proceeds and then disappeared, leaving millions of small investors stuck with losses.

The SEC and other regulators later brought cases against Wall Street's biggest banks for a number of practices designed to generate those eye-popping returns.

Recently, several hot-button technology companies and Chinese firms have generated big first-day gains.

Shares of Renren Inc , one of the biggest social networking companies in China, last month surged 296 percent in their debut. Strong demand for the unprofitable company was viewed as a sign that investors were eager to snap up stock in social media companies.

There was also a 134 percent jump by Qihoo 360 Technology in March. Yandex NV , known as Russia's Google, rose 55 percent in its trading debut.

Internet radio provider Pandora Media Inc
rose by half on its first day, though its shares fell hard in subsequent days and remain below their offering price.

Regulation M is a set of SEC rules intended to preclude manipulative conduct by individuals with an interest in the outcome of a securities offering.

Investment bankers, serving as the bridge between investors and companies, typically try to price an IPO so that the stock rises about 15 percent on the first day of trading: enough to reward investors who made a bet but not so much that issuers feel short-changed.

(Reporting by Joseph A. Giannone, additional reporting by Clare Baldwin; editing by Gerald E. McCormick, Bernard Orr)