diff --git a/doc/manual/nix-build.xml b/doc/manual/nix-build.xml
index 6640898b4..61b59c1e0 100644
--- a/doc/manual/nix-build.xml
+++ b/doc/manual/nix-build.xml
@@ -42,6 +42,7 @@
       <option>--run-env</option>
       <arg><option>--command</option> <replaceable>cmd</replaceable></arg>
       <arg><option>--exclude</option> <replaceable>regexp</replaceable></arg>
+      <arg><option>--pure</option></arg>
     </arg>
     <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
   </cmdsynopsis>
@@ -147,7 +148,10 @@ also <xref linkend="sec-common-options" />.</phrase></para>
 
     <listitem><para>In the environment of the derivation, run the
     shell command <replaceable>cmd</replaceable> instead of starting
-    an interactive shell.</para></listitem>
+    an interactive shell.  However, if you end the shell command with
+    <literal>return</literal>, you still get an interactive shell.
+    This can be useful for doing any additional
+    initialisation.</para></listitem>
 
   </varlistentry>
 
@@ -159,6 +163,21 @@ also <xref linkend="sec-common-options" />.</phrase></para>
 
   </varlistentry>
 
+  <varlistentry><term><option>--pure</option></term>
+
+    <listitem><para>If this flag is specified, the environment is
+    almost entirely cleared before the interactive shell is started,
+    so you get an environment that more closely corresponds to the
+    “real” Nix build.  A few variables, in particular
+    <envar>HOME</envar>, <envar>USER</envar> and
+    <envar>DISPLAY</envar>, are retained.  Note that
+    <filename>~/.bashrc</filename> and (depending on your Bash
+    installation) <filename>/etc/bashrc</filename> are still sourced,
+    so any variables set there will affect the interactive
+    shell.</para></listitem>
+
+  </varlistentry>
+
 </variablelist>
 
 </refsection>
@@ -189,6 +208,14 @@ $ buildPhase
 $ ./pan/gui/pan
 </screen>
 
+To clear the environment first, and do some additional automatic
+initialisation of the interactive shell:
+
+<screen>
+$ nix-build '&lt;nixpkgs>' --run-env -A pan --pure \
+    --command 'export NIX_DEBUG=1; export NIX_CORES=8; return'
+</screen>
+
 </para>
 
 <para>If a derivation has multiple outputs,
diff --git a/scripts/nix-build.in b/scripts/nix-build.in
index fb70331bf..899882952 100755
--- a/scripts/nix-build.in
+++ b/scripts/nix-build.in
@@ -10,6 +10,7 @@ use File::Temp qw(tempdir);
 my $dryRun = 0;
 my $verbose = 0;
 my $runEnv = 0;
+my $pure = 0;
 
 my @instArgs = ();
 my @buildArgs = ();
@@ -134,6 +135,10 @@ for (my $n = 0; $n < scalar @ARGV; $n++) {
         push @envExclude, $ARGV[$n];
     }
 
+    elsif ($arg eq "--pure") {
+        $pure = 1;
+    }
+
     elsif (substr($arg, 0, 1) eq "-") {
         push @buildArgs, $arg;
     }
@@ -169,6 +174,14 @@ foreach my $expr (@exprs) {
             or die "$0: failed to build all dependencies\n";
 
         # Set the environment.
+        if ($pure) {
+            foreach my $name (keys %ENV) {
+                next if $name eq "HOME" || $name eq "USER" || $name eq "LOGNAME" || $name eq "DISPLAY" || $name eq "PATH";
+                delete $ENV{$name};
+            }
+            # NixOS hack: prevent /etc/bashrc from sourcing /etc/profile.
+            $ENV{'__ETC_PROFILE_SOURCED'} = 1;
+        }
         $ENV{'NIX_BUILD_TOP'} = $ENV{'TMPDIR'} || "/tmp";
         $ENV{$_} = $drv->{env}->{$_} foreach keys %{$drv->{env}};
 
@@ -180,9 +193,9 @@ foreach my $expr (@exprs) {
         writeFile(
             $rcfile,
             '[ -e ~/.bashrc ] && source ~/.bashrc; ' .
-            'p=$PATH; ' .
+            ($pure ? '' : 'p=$PATH; ' ).
             '[ -e $stdenv/setup ] && source $stdenv/setup; ' .
-            'PATH=$PATH:$p; ' .
+            ($pure ? '' : 'PATH=$PATH:$p; ') .
             'set +e; ' .
             'PS1="\n\[\033[1;32m\][nix-build:\w]$\[\033[0m\] "; ' .
             $envCommand);