news.texi          |  4 ++++
 pygost/__init__.py |  2 +-
 pygost/mgm.py      |  2 +-
 pygost/test_mgm.py | 33 ++++++++++++++++++++++-----------

diff --git a/news.texi b/news.texi
index 2da75d59226f18d95269e6fe55f4c8b9798768bc889b9ba2abddabebf081910d..4ddfbb8899b0b522936e5c72677dcf77683e846f93852468fbc8e18b68f5649c 100644
--- a/news.texi
+++ b/news.texi
@@ -3,6 +3,10 @@ @unnumbered News
 
 @table @strong
 
+@anchor{Release 5.7}
+@item 5.7
+Fixed MGM ignoring of the set tag size.
+
 @anchor{Release 5.6}
 @item 5.6
 Fixed lint errors for previous release.
diff --git a/pygost/__init__.py b/pygost/__init__.py
index 7f8e5d3a542dc697e328e0ed0b05a527977f7d3f6adc49215769efab4e171339..2aff94fa8cab3238f06864a162679ea936adb6eda1286bc55e762036f8ff5ecc 100644
--- a/pygost/__init__.py
+++ b/pygost/__init__.py
@@ -3,4 +3,4 @@
 PyGOST is free software: see the file COPYING for copying conditions.
 """
 
-__version__ = "5.6"
+__version__ = "5.7"
diff --git a/pygost/mgm.py b/pygost/mgm.py
index 35dd4f7efaa67907ab357d45b717d0adbe7d73628e1daa17dd9d27af86d1ceab..879f05184d02c8de78875208292c06ca423e652749036c3414b9bc508e32d758 100644
--- a/pygost/mgm.py
+++ b/pygost/mgm.py
@@ -58,7 +58,7 @@                              (defaults to blocksize if not specified)
         """
         if bs not in (8, 16):
             raise ValueError("Only 64/128-bit blocksizes allowed")
-        self.tag_size = bs if tag_size is None else bs
+        self.tag_size = bs if tag_size is None else tag_size
         if self.tag_size < 4 or self.tag_size > bs:
             raise ValueError("Invalid tag_size")
         self.encrypter = encrypter
diff --git a/pygost/test_mgm.py b/pygost/test_mgm.py
index 124449b6bdd670be2c9b43b7f974631962bf3820261355a49860a0a91f564dda..6762a411209096f3d783191311b1c8d736231cda961b705cae995e9cfb0daf95 100644
--- a/pygost/test_mgm.py
+++ b/pygost/test_mgm.py
@@ -39,7 +39,7 @@         self.assertSequenceEqual(mgm.open(plaintext[:16], ciphertext, ad), plaintext)
 
 
 class TestSymmetric(TestCase):
-    def _itself(self, mgm, bs):
+    def _itself(self, mgm, bs, tag_size):
         for _ in range(1000):
             nonce = nonce_prepare(urandom(bs))
             ad = urandom(randint(0, 20))
@@ -47,18 +47,29 @@             pt = urandom(randint(0, 20))
             if len(ad) + len(pt) == 0:
                 continue
             ct = mgm.seal(nonce, pt, ad)
+            self.assertEqual(len(ct) - tag_size, len(pt))
             self.assertSequenceEqual(mgm.open(nonce, ct, ad), pt)
 
     def test_magma(self):
-        mgm = MGM(
-            GOST3412Magma(urandom(KEYSIZE)).encrypt,
-            GOST3412Magma.blocksize,
-        )
-        self._itself(mgm, GOST3412Magma.blocksize)
+        for tag_size in (
+                GOST3412Magma.blocksize,
+                GOST3412Magma.blocksize - 2,
+        ):
+            mgm = MGM(
+                GOST3412Magma(urandom(KEYSIZE)).encrypt,
+                GOST3412Magma.blocksize,
+                tag_size,
+            )
+            self._itself(mgm, GOST3412Magma.blocksize, tag_size)
 
     def test_kuznechik(self):
-        mgm = MGM(
-            GOST3412Kuznechik(urandom(KEYSIZE)).encrypt,
-            GOST3412Kuznechik.blocksize,
-        )
-        self._itself(mgm, GOST3412Kuznechik.blocksize)
+        for tag_size in (
+                GOST3412Kuznechik.blocksize,
+                GOST3412Kuznechik.blocksize - 2,
+        ):
+            mgm = MGM(
+                GOST3412Kuznechik(urandom(KEYSIZE)).encrypt,
+                GOST3412Kuznechik.blocksize,
+                tag_size,
+            )
+            self._itself(mgm, GOST3412Kuznechik.blocksize, tag_size)