diff --git a/sknn/backend/lasagne/mlp.py b/sknn/backend/lasagne/mlp.py
index 2326535..7d90a9b 100644
--- a/sknn/backend/lasagne/mlp.py
+++ b/sknn/backend/lasagne/mlp.py
@@ -234,12 +234,10 @@ def cast(array):
         for start_idx in range(0, total_size - batch_size + 1, batch_size):
             excerpt = indices[start_idx:start_idx + batch_size]
             Xb, yb = cast(X[excerpt]), cast(y[excerpt])
-            if self.mutator is not None:
-                for x, _ in zip(Xb, yb):
-                    self.mutator(x)
+
             yield Xb, yb
 
-    def _batch_impl(self, X, y, processor, output, shuffle):
+    def _batch_impl(self, X, y, processor, mode, output, shuffle):
         progress, batches = 0, X.shape[0] / self.batch_size
         loss, count = 0.0, 0
         for Xb, yb in self._iterate_data(X, y, self.batch_size, shuffle):
@@ -255,10 +253,10 @@ def _batch_impl(self, X, y, processor, output, shuffle):
         return loss / count
         
     def _train_impl(self, X, y):
-        return self._batch_impl(X, y, self.trainer, output='.', shuffle=True)
+        return self._batch_impl(X, y, self.trainer, mode='train', output='.', shuffle=True)
 
     def _valid_impl(self, X, y):
-        return self._batch_impl(X, y, self.validator, output=' ', shuffle=False)
+        return self._batch_impl(X, y, self.validator, mode='valid', output=' ', shuffle=False)
 
     @property
     def is_initialized(self):
diff --git a/sknn/nn.py b/sknn/nn.py
index 1d78257..015cd41 100644
--- a/sknn/nn.py
+++ b/sknn/nn.py
@@ -342,11 +342,6 @@ class NeuralNetwork(object):
         only be applied to layers of type ``Linear`` or ``Gaussian`` and they must be used as
         the output layer (PyLearn2 only).
 
-    mutator: callable, optional
-        A function that takes a single training sample ``(X, y)`` at each epoch and returns
-        a modified version.  This is useful for dataset augmentation, e.g. mirroring input
-        images or jittering.
-        
     callback: callable or dict, optional
         An observer mechanism that exposes information about the inner training loop. This is
         either a single function that takes ``cbs(event, **variables)`` as a parameter, or a
@@ -405,7 +400,6 @@ def __init__(
             valid_set=None,
             valid_size=0.0,
             loss_type=None,
-            mutator=None,
             callback=None,
             debug=False,
             verbose=None,
@@ -457,7 +451,6 @@ def __init__(
         self.valid_set = valid_set
         self.valid_size = valid_size
         self.loss_type = loss_type
-        self.mutator = mutator
         self.debug = debug
         self.verbose = verbose
         self.callback = callback
diff --git a/sknn/tests/test_data.py b/sknn/tests/test_data.py
index 26de705..b132f6e 100644
--- a/sknn/tests/test_data.py
+++ b/sknn/tests/test_data.py
@@ -17,12 +17,12 @@ def setUp(self):
         self.nn = MLPR(
                     layers=[L("Linear")],
                     n_iter=1,
-                    batch_size=2,
-                    mutator=self._mutate_fn)
+                    batch_size=1,
+                    callback={'on_batch_start': self._mutate_fn})
 
-    def _mutate_fn(self, sample):
+    def _mutate_fn(self, Xb, **_):
         self.called += 1
-        sample[sample == 0.0] = self.value
+        Xb[Xb == 0.0] = self.value
 
     def test_TestCalledOK(self):
         a_in, a_out = numpy.zeros((8,16)), numpy.zeros((8,4))
diff --git a/sknn/tests/test_types.py b/sknn/tests/test_types.py
index 77da0ea..6b38f86 100644
--- a/sknn/tests/test_types.py
+++ b/sknn/tests/test_types.py
@@ -42,10 +42,10 @@ def test_FitHybrid(self):
             self.nn._fit(X, y)
 
     def test_FitMutator(self):
-        def mutate(x):
+        def mutate(Xb, **_):
             self.count += 1
-            return x - 0.5
-        self.nn.mutator = mutate
+            Xb -= 0.5
+        self.nn.callback = {'on_batch_start': mutate}
 
         for t in SPARSE_TYPES:
             sparse_matrix = getattr(scipy.sparse, t)