The device had been left on a window sill of a house in Holywood in Co Down at around midnight.
No-one was injured in the attack.
Army bomb disposal experts and police examined the scene and took a number of items for forensic investigation.
Incidents in Belfast
A number of windows were broken in homes on the Whitewell Road in North Belfast last night. A floral memorial to a deceased teenager was damaged.
The trouble took place just before midnight and police are to examine closed circuit television tapes later today.
Police are also investigating a gun and knife attack outside Belfast just before midnight.
It is understood a car containing several people pulled up outside a house on Brian's Well Road in the Dunmurray area.
A man got out and entered the house, slashing a woman who was inside with a knife.
A shot was then fired through a front window.
The woman's injuries are not thought to be life-threatening.
