Some spammers are dipping into ancient tricks to slip their junk mail past filters, a security firm said Tuesday.

In the days when monitors displayed only text, not graphics, enterprising computer users would assemble images by placing characters on the screen.

Researchers at U.K.-based Sophos have spotted some spammers using the so-called " ASCII art" to create large-sized words touting their products, such as prescription drugs.

"By using old-fashioned ASCII art with carefully positioned randomly chosen letters, spammers are attempting to sneak past e-mail filters hunting for phrases commonly used in spam," said Graham Cluley, a senior technology consultant for Sophos, in a statement.

"It's a sneaky trick. The spam e-mail is advertising drugs like 'Viagra' and 'Cialis' without having to use the actual words."