I hope your file looks rather like this:
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback
# The primary network interface
# allow-hotplug eth1
iface eth1 inet static
The leading spaces are important - they mean that the address, netmask, etc. settings still belong to eth1.