Dissolve yeast in warm water.  In a large bowl combine milk, sugar, and salt.  Use a mixer to beat in molasses, butter, yeast mixture and 1 cup of rye flour.  Use a wooden spoon to mix in the remaining rye flour.  Add white flour by stirring until the dough is stiff enough to knead.  Knead 5 to 10 minutes, adding flour as needed.  If the dough sticks to your hands or the board add more flour.  Cover dough and let rise 1 1 1/2 hours or until double.  Punch down dough and divide to form 2 round loaves. 
Let loaves rise on a greased baking sheet until double, about 1 1/2 hours. 
Preheat oven to 375 degrees F.  Bake for 30 to 35 minutes.