With almost the kind of secrecy that once shrouded the development of the Stealth bomber, M&M Mars Co. is quietly test-marketing a new peanut butter version of its M&M's candies.

The candy company is refusing to say where consumers can find the new version of the treats that "melt in your mouth, not in your hand.""Two small test markets on the East Coast," was all company spokesman Hans Fiuczynski would say about the locations in a telephone interview Thursday.

He said the new "M&M Chocolate and Peanut Butter candies" have been available in the test markets for about five weeks, but refused to identify the towns, saying it was the company's standard practice on such experimental products.

The new lentil-shaped version of M&M's is said to resemble the traditional peanut version, except that the peanut within the candy and chocolate shell is replaced by peanut butter.

Fiuczynski said the company's reasons for secrecy are competition and public reaction.

Ever since an advertising industry magazine mentioned the new candies about two weeks ago, Fiuczynski said, he has been flooded with letters from consumers trying to learn where, when and how they can get them.

"I've had my hands full . . . We don't like to unnecessarily frustrate consumers," he said.