The 26-year-old man copied down his mother's credit card numbers without her knowledge and charged for gasoline and motel rooms that in one month totaled $793.43.

When his mother received the bill, she immediately filed a protest with the bank, saying that the charges had been made without her authorization. She asked that they be taken off her bill, in keeping with the Fair Credit Billing Act, the federal law that relieves the cardholder of responsibility for unauthorized purchases exceeding $50.

"They told me I had to file charges against him or I had to pay the bill," said the mother, a 51-year-old school teacher who lives in Springfield.

But the woman, unconvinced that she is liable, is trying now to get an answer from the federal agencies that have jurisdiction over institutions that issue credit cards. Because of the circumstances of this case, it isn't clear what the final outcome may be, and the case may have to be decided in court.

Meanwhile, in response to questions about the credit law, the Federal Trade Commission said that parents are responsible for unauthorized charges by their minor children, but depending on the circumstances, they may or may not be responsible for unauthorized charges by children who have reached the age of majority, which in most states is 18.

The fact that the young man used the numbers from the credit card rather than the card itself to make the purchases strengthens his mother's argument that she is not liable for the charges, according to Sarah Hughes, an FTC attorney specializing in credit law.

"When the person making the charge doesn't have a card in his possession and is simply using the numbers from the card, the merchant is the one running the risk of being liable rather than the cardholder," she said.

"It is up to the merchant to check for authorization when the amount of the purchase exceeds a certain level and when the customer has no card with him," she said. "What may be wrong here is that the merchant may have failed to follow proper procedures in honoring the card. And if so, he would be liable."

But the key to the case, Hughes said, is whether the 26-year-old has used his mother's card in the past. "If so, then he may have had the apparent authority to use the card [to charge the $793.4]," she said, and that could make the mother liable.

The mother said her son hadn't used her card before the one month's spending binge. Moreover, she said her own monthly card charge averages $25 a month or less--never as much as $793.43.

Her story is typical of the dilemma of the credit cardholder who knows he or she has certain protections under the law but isn't quite sure what they are or how to go about asserting them. In this case, the mother followed the proper procedures for disputing the bill, Hughes said. That includes these steps:

* Call immediately to notify the company that there are unauthorized charges on your bill.

* File a written notice with the company within 60 days of receiving the bill with the unauthorized charges. Send it to the address printed on the back of the periodic statement. Be sure to include your name, address and account number, the nature of the problem, the amount in dispute and why it is in dispute.

* Remember that as long as the charges are in dispute, you are not liable for interest charges on the disputed amount.

If the company investigates and concludes that you still are liable for the charges on your bill, and you still contend that you are not, you can go to the federal agency with jurisdiction over that company's billing practices.

The four agencies that share that responsibility include:

* The FTC, which has jurisdiction over credit cards issued by retail department stores and companies such as American Express and Diners Club International but not over cards issued by banks. Call 724-1139 for FTC credit information.

* The Federal Deposit Insurance Corp., which has jurisdiction over credit cards issued by state-chartered banks that aren't members of the Federal Reserve System. Call the consumer hot line at 389-4767.

* The Federal Reserve Board, which has jurisdiction over credit cards issued by state-chartered banks that are members of the Federal Reserve System. Write the Division of Consumer and Community Affairs, Federal Reserve Board, Washington, D.C. 20551.

* The Comptroller of the Currency, which has jurisdiction over credit cards issued by the national banks. Call 447-1600 for consumer information.