In a heavy-bottomed 4-quart saucepan, heat milk, stirring frequently over medium heat, to 185 degrees. Remove from heat, and let cool to 110 degrees.
Place yogurt in a medium bowl. Using a whisk, gradually stir in cooled milk, about 1/2 cup at a time, stirring until smooth between additions. Cover the bowl with plastic wrap, poking two or three holes for ventilation. Transfer to a warm place (about 90 degrees), and let sit until milk begins to thicken around the edges and the yogurt is set, about 5 hours.
Place bowl in refrigerator until completely chilled. Reserve at least 1/4 cup of this yogurt to begin next batch.