Great info on kefir and yogurt:

http://en.wikipedia.org/wiki/Kefir

http://users.chariot.net.au/~dna/kefirpage.html

http://www.thenibble.com/reviews/nut...iotic-food.asp

Make sure your yogurt says it contains LIVE, ACTIVE cultures. If it's been pasteurized, you're out of luck - the good bacteria (probiotics) you're after have been killed off.