diff --git a/README b/README
index 3aa79d23..63835bdb 100644
--- a/README
+++ b/README
@@ -99,6 +99,12 @@ Unit tests are written using py.test, and its pytest-django support library.
 
   DJANGO_SETTINGS_MODULE=combo.settings COMBO_SETTINGS_FILE=tests/settings.py py.test
 
+Tests for w.c.s. cells do require access to the wcsctl script, its location has
+to be given in a WCSCTL environment variable, this give a full command line:
+
+  WCSCTL=$(pwd)/wcs/wcsctl.py \
+  DJANGO_SETTINGS_MODULE=combo.settings COMBO_SETTINGS_FILE=tests/settings.py py.test
+
 
 License
 -------