diff --git a/lib/string_tools/core_ext/string.rb b/lib/string_tools/core_ext/string.rb
index 0089422..9b1c956 100644
--- a/lib/string_tools/core_ext/string.rb
+++ b/lib/string_tools/core_ext/string.rb
@@ -1,3 +1,5 @@
+# coding: utf-8
+
 require 'rchardet19'
 require 'addressable/uri'
 require 'active_support/core_ext/module'
@@ -192,6 +194,15 @@ def to_cp1251!
     self.replace(self.to_cp1251)
   end
 
+  def mb_downcase
+    # https://github.com/rails/rails/commit/393e19e508a08ede0f5037bccb984e3eb252d579
+    if ActiveSupport::VERSION::STRING >= '4.0.0' && ActiveSupport::VERSION::STRING <= '4.2.0'
+      ActiveSupport::Multibyte::Unicode.send(:database).codepoints
+    end
+
+    mb_chars.downcase.to_s
+  end
+
   private
 
   def surround_with_ansi(ascii_seq)
diff --git a/spec/string_spec.rb b/spec/string_spec.rb
new file mode 100644
index 0000000..23d584d
--- /dev/null
+++ b/spec/string_spec.rb
@@ -0,0 +1,9 @@
+# coding: utf-8
+
+require 'spec_helper'
+
+describe String do
+  describe '#mb_downcase' do
+    it { expect("Кириллица".mb_downcase).to eq("кириллица") }
+  end
+end