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Mr. Yuk is a poison awareness symbol; he is a neon-green green cartoon face sticking out his tongue in disgust which appears on stickers to warn young children that a substance is poisonous or dangerous, and also to keep the phone number for poison control handy in case someone does ingest something dangerous. Most American Mr. Yuk stickers now contain the name of the nearest poison control center and a toll-free nationwide Poison Control Hotline (1-800-222-1222). The symbol has also been used in other countries; I don't know if those stickers contain local phone numbers or have a similar widespread number.

Mr. Yuk was created at the Poison Center at Children's Hospital of Pittsburgh in 1971 to replace the skull and crossbones symbol previously used to mark poisons, since it is frequently used "to convey happy, exciting things like pirates and adventure." The symbol has found its way into textbooks, substance packaging, posters, and the National Capital Poison Center even sells Mr. Yuk T-shirts on its web site. But many poison control centers will send out the regular stickers for free or the cost of postage and handling to anyone who requests them. However, some poison centers have stopped using the logo and stickers on the grounds that the bright green image attracts rather than scares children. (The point was always to tell children that the symbol meant poison, with all the commercials and school presentations.)

And he came with his own theme song (at least in the late 1970s when I first heard awareness commercials using the symbol).

"Mr. Yuk is mean,
Mr. Yuk is green."

Apparently these commercials are no longer broadcast, but they made quite an impression on those who saw/heard them.

There is also a rock band using the name Mr. Yuk; the band is from Seattle, Washington .

Sources:
http://www.chp.edu/mryuk/05a_mryuk.php
http://www.thepittsburghchannel.com/news/1192325/detail.html
http://www.tvparty.com/wwwboard/messages/722.html
http://silverchips.mbhs.edu/inside.php?sid=1918
http://www.in.gov/fssa/children/bcd/goodbyemryuk0304.html
http://www.killrockstars.com/bands/factsheets/mryuk/