Legg Mason Inc., the big Baltimore-based stock broker, disclosed plans yesterday to sell its own stock to the public for the first time.

The brokerage firm filed a registration statement with the Securities and Exchange Commission to sell about 18 percent of the firm to the public for approximately $15 million.

In addition to the 800,000 shares to be sold by the company itself, a group of Legg Mason's present owners plans to sell another 660,000 shares worth about $12 million.

Legg Mason's officers and directors will still control more than 65 percent of the company's shares after the offer is completed. No date was disclosed for the sale of the stock, which was priced at a maximum of $19 a share.

The principal business of Legg Mason Inc. is Legg Mason Wood Walker Inc., a major Middle Atlantic regional securities firm. Legg Mason Wood Walker has 28 offices now and two more about to open.

The cash raised by the sale of stock to the public will "be used to expand the securities brokerage business" and "for continued diversification into related financial service businesses," the announcement said.

The prospectus for the new shares was not made public yesterday and Legg Mason officials said they could not elaborate on their plans until the formal offer is available.