This article is from the Care of Orphaned Kittens FAQ, posted to rec.pets.cats newsgroup. Maintained by Sharon Talbert with numerous contributions by others.
Any kitten, even if it seems fine, should be seen by your vet as soon as possible. Take a fresh stool sample with you, so the vet may check for intestinal parasites. If a fecal cannot be done by the time of the appointment, take a stool sample as soon as you can. Remove as many fleas as you safely can (with a comb for newborns; do not bathe, spray or powder a kitten before it is six weeks old).
If the kitten is lethargic or cool to the touch, you may have a life-threatening emergency (such as exposure or distemper). Get the kitten on a heating pad or other primary heat source (see item 2) and get it to a veterinarian right away or consult an emergency veterinary clinic. Do NOT feed a chilled newborn -- you will kill it. Instead, administer slightly warmed Pedialyte (an infant rehydrating fluid, available in any grocery or pharmacy), using an animal nurser, syringe, or dropper. (You can greatly extend the life of the Pedialyte by freezing it as ice cubes, bagging the cubes and storing them in your freezer, by the way.) Feed the kitten only when it is warmed and indicates it is hungry.
If the kitten seems over-warm and/or is breathing rapidly, it may be feverish or suffering from heat exhaustion or worse. Contact your vet or an emergency veterinary clinic immediately for advice if you can. To help lower the kitten's body temperature, try wiping it down with a cool, damp cloth; then administer Pedialyte. Get the kitten to a veterinarian as soon as possible.
If the kitten is active and screaming lustily for its mother, go quickly to item 2; you will find that a heating pad will help calm the kitten while you prepare its first meal.