diff --git a/xblock/fields.py b/xblock/fields.py
index 8f5eecc8a..ee037581e 100644
--- a/xblock/fields.py
+++ b/xblock/fields.py
@@ -307,7 +307,6 @@ class will want to refer to.
             runtime_options.
 
     """
-    MUTABLE = True
     _default = None
     # Indicates if a field's None value should be sent to the XML representation.
     none_to_xml = False
@@ -334,10 +333,7 @@ def __init__(self, help=None, default=UNSET, scope=Scope.content,  # pylint:disa
     @property
     def default(self):
         """Returns the static value that this defaults to."""
-        if self.MUTABLE:
-            return copy.deepcopy(self._default)
-        else:
-            return self._default
+        return copy.deepcopy(self._default)
 
     @property
     def name(self):
@@ -516,10 +512,7 @@ def __get__(self, xblock, xblock_class):
                 value = self.default
             self._set_cached_value(xblock, value)
 
-        # If this is a mutable type, mark it as dirty, since mutations can occur without an
-        # explicit call to __set__ (but they do require a call to __get__)
-        if self.MUTABLE:
-            self._mark_dirty(xblock, value)
+        self._mark_dirty(xblock, value)
 
         return self._sanitize(value)
 
@@ -701,7 +694,6 @@ class Integer(JSONField):
     containing a floating point number ('3.48') will throw an error.
 
     """
-    MUTABLE = False
 
     def from_json(self, value):
         if value is None or value == '':
@@ -720,7 +712,6 @@ class Float(JSONField):
     something for which float(value) does not throw an error.
 
     """
-    MUTABLE = False
 
     def from_json(self, value):
         if value is None or value == '':
@@ -750,7 +741,6 @@ class Boolean(JSONField):
         None - > False
 
     """
-    MUTABLE = False
 
     # We're OK redefining built-in `help`
     def __init__(self, help=None, default=UNSET, scope=Scope.content, display_name=None,
@@ -853,7 +843,6 @@ class String(JSONField):
     The value, as loaded or enforced, can either be None or a basestring instance.
 
     """
-    MUTABLE = False
     BAD_REGEX = re.compile('[\x00-\x08\x0b\x0c\x0e-\x1f\ud800-\udfff\ufffe\uffff]', flags=re.UNICODE)
 
     def _sanitize(self, value):
diff --git a/xblock/reference/plugins.py b/xblock/reference/plugins.py
index 17825942e..2e9b28e4b 100644
--- a/xblock/reference/plugins.py
+++ b/xblock/reference/plugins.py
@@ -122,7 +122,6 @@ class Filesystem(Field):
     src=...> will typically be faster through this than serving that
     up through XBlocks views.
     """
-    MUTABLE = False
 
     def __get__(self, xblock, xblock_class):
         """
diff --git a/xblock/test/test_fields.py b/xblock/test/test_fields.py
index 6f67f71eb..850563caf 100644
--- a/xblock/test/test_fields.py
+++ b/xblock/test/test_fields.py
@@ -703,7 +703,6 @@ def test_setting_the_same_value_marks_field_as_dirty():
     # pylint: disable=unsubscriptable-object
     class FieldTester(XBlock):
         """Test block for set - get test."""
-        non_mutable = String(scope=Scope.settings)
         list_field = List(scope=Scope.settings)
         dict_field = Dict(scope=Scope.settings)
 
@@ -714,17 +713,13 @@ class FieldTester(XBlock):
     assert len(field_tester._dirty_fields) == 0
     assert not field_tester.fields['list_field'].is_set_on(field_tester)
     assert not field_tester.fields['dict_field'].is_set_on(field_tester)
-    assert not field_tester.fields['non_mutable'].is_set_on(field_tester)
 
-    field_tester.non_mutable = field_tester.non_mutable
     field_tester.list_field = field_tester.list_field
     field_tester.dict_field = field_tester.dict_field
 
-    assert not field_tester.fields['non_mutable'] in field_tester._dirty_fields
     assert field_tester.fields['list_field'] in field_tester._dirty_fields
     assert field_tester.fields['dict_field'] in field_tester._dirty_fields
 
-    assert not field_tester.fields['non_mutable'].is_set_on(field_tester)
     assert not field_tester.fields['list_field'].is_set_on(field_tester)
     assert not field_tester.fields['dict_field'].is_set_on(field_tester)