young Affen started biting visitors

Question
My year and a half old neutered Affen has always barked unceasingly at
visitors to my house, but yesterday it got worse--he bit a visitor. Today he
did it again. Neither bite broke skin, but I know I've got to do something
about this IMMEDIATELY. He's in his second obedience class, where he does
well, in spite of being pretty fearful.

Answer
This is called territorial/fearful aggression, and is common in dogs that are under- or un-socialized to a lot of visitors during puppy hood, and in some high strung breeds.  Some behavior specialists advocate low protein diets and tryptophan supplementation for such dogs, some recommend medications such as Clomicalm, but most feel that the situation must also involve some level of behavior modification (classical or counter-conditioning exercises).  I would suggest finding a behaviorist near you and making an appointment.  You can find one at the Animal Behavior Society web site, or at IAABC.com.  In the meantime, you must protect your guests, so don't be afraid to get him acclimated to wearing a muzzle.  Morrco.com has them, plus a nice handout on how to acclimate the dog to wearing one.  Good luck.