Combine milk and half and half in a saucepan. Bring to a boil over low heat. Remove from heat and transfer to a clean bowl. Set aside to cool to 115 degrees F. Add yogurt and vigorously whisk.
Cover bowl with plastic, then wrap well with heavy towels or a blanket. Set aside in a warm place for 6 to 8 hours, or longer according to taste. The longer yogurt sits, the more acidic it will become. Store in sealed containers in refrigerator.
c.1997, M.S. Milliken & S. Feniger, all rights reserved