This article is from the Lego FAQ, by Tom Pfeifer firstname.lastname@example.org with numerous contributions by others.
From a LEGO catalog...
DUPLO and LEGO SYSTEM toys can be washed by hand, using warm
water -- max. 104 degrees Fahrenheit (40 Celsius) -- and a mild
liquid dish detergent. Storage temperature max. 104 degrees
Fahrenheit = 40 Celsius. Electric parts are not washable.
email@example.com (Juergen Christoffel) and firstname.lastname@example.org (Jack Gilmer)
Put your LEGO bricks into a pillow case or a mesh bag (the kind
for washing small articles of clothes) and wash in your washing
machine at a low temperature. Tested in kindergarten once a
year. (Be sure to put no metal or electric parts into the
machine, and wash clear pieces seperately by hand)
email@example.com (Alekz Vermont) says:
stick them in the tub w/warm sudsy water and swish about... let
soak. swish more. drain tub. spray with shower (to rinse) and
Do not wash your LEGO people -- their faces come off!
... but firstname.lastname@example.org (Meng Soo) notes:
There's nothing wrong with that. I'd pretend that their faces
melted, and became faceless mutant LEGO people. The fun really
started when I discovered permanent markers...