Patrick Combs recently performed an experiment that any medieval alchemist would aspire to: He turned a piece of junk mail into cold, hard cash.

The only problem is, he performed the experiment on a bank, and the bank wants its money back.

Mr. Combs, a 29-year-old San Francisco author, was one of 40 million Americans who recently received a fake check in the mail as part of a promotional campaign by the Association of Certified Liquidators, a company that buys and resells stores' unsold goods.

Unlike the other recipients, however, Mr. Combs deposited the $95,093.35 ``check'' in his account at Los Angeles-based First Interstate Bancorp, as a lark. He just wanted to see what would happen, he says.

``I was absolutely certain the bank wouldn't cash it,'' says Mr. Combs.

But lo and behold, the bank credited the entire sum to his account by May 22, the next business day.

Mr. Combs waited three weeks, expecting the bank to discover its error. When it didn't, he withdrew every last penny and put a cashier's check for the full amount in his safe-deposit box at the bank _ and he has a deposit slip and copy of the cashier's check to prove it.

``There's no way I'd leave the bank with that much money,'' he says. ``If fate can give it to me, it can take it away just as fast.''

The bank now hopes to do just that. A week after he withdrew the money, Mr. Combs discovered _ when an automated teller machine ate his bank card _ that his account had been frozen, he says. He also received what he describes as threatening messages on his home answering-machine from bank representatives ordering him to give back the loot. Mr. Combs refused. He says he would have returned the money if the bank had been more polite.

``I would have liked the bank to just call me and admit it erred,'' says Mr. Combs. ``And maybe take me to lunch.''

First Interstate, citing customer confidentiality, declines to comment on the case.

ACL President Mitch Klass is surprised by the financial uproar caused by his company's errant check.

``No one has ever tried to cash one of our promotional checks before,'' he says. ``I didn't even know it this time. We're going to have to check into this.''