Dissolve yeast in water. Pour milk over the salt, molasses, & butter. When cooled to lukewarm add the yeast mixture.
Beat in the white flour. Beat until very smooth. Gradually add the rye flour and mix to make a medium stiff dough (it will be sticky). Knead dough until smooth. Place in bowl and cover. Let rise until doubled - about 2 hours.
Punch down and let rise again. Turn out and shape into 2 oblong loaves. Place on greased baking sheet. Let rise until doubled. Bake at 375º F for 30 to 40 minutes.