The M67 Frag Grenade is a small explosive object that will explode shortly after being thrown. Its main use is killing or injuring players, and is poorly suited to dealing with zombies. To equip the grenade, press the "F" key until "Grenade" is displayed in the top right corner of the screen. You can then throw it using the left-mouse button. Holding down the button will cause the grenade to be thrown further.
Other hand thrown grenades include Smoke Grenade, smoke-grenade-green, and smoke-grenade-red. The M16A2 M203 and M4A1 Holo have secondary M203 Grenade Launcher attachments capable of firing M203 HE, M203 Smoke, M203 Flare White, and M203 Flare Green.
An explosion that is less than 10 meters away will usually be fatal. At less than 20 meters, the grenade can break bones, cause bleeding, deafen and knock players unconscious. It is useful for greedy players that want to blow up vehicles they can't use or do not need.
Another variant of grenade is the RGO Anti-personnel timed hand grenade. These will detonate on contact with any surface, meaning great care must be taken and using them while inside a building is likely to result in death from either the explosion or the collapsing of the building. Aside from this, they are identical. 10 RGO grenades are found in newly-spawned UAZs, along with other fairly valuable loot. They cannot be found elsewhere.