diff --git a/aiohttp/payload.py b/aiohttp/payload.py
index e21b7bda72d..cab72a2eb84 100644
--- a/aiohttp/payload.py
+++ b/aiohttp/payload.py
@@ -3,6 +3,7 @@
 import json
 import mimetypes
 import os
+import warnings
 from abc import ABC, abstractmethod
 
 from multidict import CIMultiDict
@@ -18,6 +19,7 @@
            'IOBasePayload', 'BytesIOPayload', 'BufferedReaderPayload',
            'TextIOPayload', 'StringIOPayload', 'JsonPayload')
 
+TOO_LARGE_BYTES_BODY = 2 ** 20
 
 class LookupError(Exception):
     pass
@@ -150,6 +152,11 @@ def __init__(self, value, *args, **kwargs):
 
         self._size = len(value)
 
+        if self._size > TOO_LARGE_BYTES_BODY:
+            warnings.warn("Sending a large body directly with raw bytes might"
+                          " lock the event loop. You should probably pass an "
+                          "io.BytesIO object instead", ResourceWarning)
+
     @asyncio.coroutine
     def write(self, writer):
         yield from writer.write(self._value)
diff --git a/tests/test_client_functional.py b/tests/test_client_functional.py
index 48bcc9a2971..3804a01d3f7 100644
--- a/tests/test_client_functional.py
+++ b/tests/test_client_functional.py
@@ -1286,6 +1286,46 @@ def handler(request):
         resp.close()
 
 
+@asyncio.coroutine
+def test_POST_bytes(loop, test_client):
+    body = b'0' * 12345
+
+    @asyncio.coroutine
+    def handler(request):
+        data = yield from request.read()
+        assert body == data
+        return web.HTTPOk()
+
+    app = web.Application()
+    app.router.add_post('/', handler)
+    client = yield from test_client(app)
+
+    resp = yield from client.post('/', data=body)
+    assert 200 == resp.status
+    resp.close()
+
+
+@asyncio.coroutine
+def test_POST_bytes_too_large(loop, test_client):
+    body = b'0' * 2 ** 17
+
+    @asyncio.coroutine
+    def handler(request):
+        data = yield from request.read()
+        assert body == data
+        return web.HTTPOk()
+
+    app = web.Application()
+    app.router.add_post('/', handler)
+    client = yield from test_client(app)
+
+    with pytest.warns(ResourceWarning):
+        resp = yield from client.post('/', data=body)
+
+    assert 200 == resp.status
+    resp.close()
+
+
 @asyncio.coroutine
 def test_POST_FILES_STR(loop, test_client, fname):
     @asyncio.coroutine