Brace, yourselves. More madness from the world of commerce.

Today's I-don't-believe-it: Some businesses won't handle $100 bills.

I know, you bright-eyed, naive soul. You thought companies were only too glad to accept whatever denomination John Q. Customer might care to tender. You though a company would be delighted to get cash any time it could, because it would thereby avoid bad-check writers, credit-card misusers and other crooks. You thought cash was cash was cash.

Well, you haven't met First American Bank of Washington or the Loehmann's chain of clothing stores.

First American won't make change for $100 bills unless you have an account at the bank.

Even though the bill says "legal tender?" "I'm afraid not," said senior vice president Francis Curtin.

"If we started doing it regularly," said Curtin, "the con artists would come in. There are an awful lot of counterfeit bills floating around. We just recently had a case where one of our tellers came up a couple of hundred short. It's just not worth it to us."

And Loehmann's? At many of their stores, you can't buy anything with a hundred-dollar bill without producing identification.

"We have no storewide policy of requiring identification [from those who present $100 bills]," said public relations director Mary Jean Rainnie. However, until two years ago, the Falls Church Loehmann's had that policy, according to assistant manager barbara Thomason. The Rockville and Towson stores have that policy today, according to their managements. And several other Loehmann's branches around the country do, too.

Why? Again, the danger of counterfeit, say the Loehmann's managers.

I have no objection to these businesses, or any others, trying to protect themselves against fraud. But federal specialists will tell you that most of the counterfeit bills passed in ths country are 20s. Second most common phony: tens. Followed in order by fives and ones.

What the First American and Loehmann's people have failed to think through is that anyone passing a $100 bill is attracting attention to himself anywa. Wouldn't a counterfeiter want to do just the opposite?