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 to 1 1/2 hours or until double.
Punch down dough and divide to form two round loaves. Let loaves rise on a greased baking sheet until double, about 1 1/2 hours.
Preheat oven to 375 degrees F (190 degrees C). Bake for 30 to 35 minutes.