If you own stocks, you may be losing money that you don't even know about. That happens when investors don't understand exactly what they've bought or are oblivious to what happens to the companies in which they own shares.

For example, you might have received a tender offer for your stock and didn't respond. Shareholder Communications Corp. estimates, conservatively, that mergers in the 1980s alone produced $150 million in tender offers that haven't been claimed.

You also may own valuable stock rights or warrants that -- unless exercised -- will expire worthless.

Here are some of the money-making offers that you shouldn't let get by:

A tender offer. A buyer wants your company and has made an offer for some or all of its shares, at something more than the market price. Sometimes a second buyer makes a higher bid. Tender offers drive up the price of your shares on the open market, although not necessarily quite as high as the tender price. A quick way to profit is simply to sell your shares.

Tendering shares to the buyer is less certain. The offer might be part cash, part preferred stock and part debt -- creating a package of uncertain value. If the deal falls apart, tenderers get their shares back, which might then be selling at a lower price.

With a partial tender, the buyer may take a portion of the shares surrendered by investors and return the rest. Once a partial tender is complete, the share price typically falls.

That drop in price makes it possible for brokers to play games. They tender customers' shares, pocket the high price, buy the shares back when the price falls and restore the securities to customer accounts. Customers are none the wiser.

What happens if a buyer tenders for all the shares and you never get around to surrendering yours? If a broker holds the certificates, the money goes to the brokerage house, which should alert you to the offer. If you don't respond within five to seven years, the money goes to the state as unclaimed property. You can get back your money from the state, but no interest is paid.

If you hold your own securities and don't respond to the offer, the company puts a stop-payment on your account after five to seven years and remits the money to the state.

Stock rights. When a company issues new shares, it might give first dibs to its current stockholders, charging them less than market price. The piece of paper that stockholders get, giving them this cut-rate deal, is called a ''right.'' If you don't exercise your rights to the cheap shares within a specified period of time, you lose them.

Warrants. When you buy certain speculative securities, such as new issues, they might come with ''warrants,'' which give you the right to buy more shares at a specified, higher price. If the market moves above that price, your warrants become especially valuable. But if you don't sell them or use them to claim the stock, they may expire worthless.

If you're not prepared to follow your investments closely enough to keep track of these offers, do yourself a favor and buy mutual funds instead.

Correction: Last week's column on checking up on your stockbroker contained an incorrect telephone number for the North American Securities Administrators Association. The correct number is (202) 737-0900.