Men in Black (MiB) and Women in Black (WiB) are physiopharmaceutically augmented agents of Majestic 12 posing as government agents early on in the game. They are much stronger than average enemies and have access to advanced weaponry. The WiB shows up later through the game.
MiBs have unnaturally pale skin and red eyes. They're said to be caused by the physiopharmaceutical augmentation process. Because of this they wear sunglasses and dark clothing to mask the albino appearance. Male MIBs are very tall, standing a full head taller than an average human male. Their voice sounds robotic. Some theorize that it's because of an integrated rebreather in their bronchus, but even if it exists, it gives them no resistance to poison and gas unlike the IET. MiBs have the symbol of the Illuminati (image of an eye) tattooed to the back of their necks.
MiBs have improved capabilities thanks to drugs with addition of physical and psychological conditioning. Their strength and endurance are greatly improved (Arthur Donovan, the lead scientist behind the project, predicts that they would be 78% more effective than unaugmented operatives).
An important feature is a self-termination mechanism wired into the carrier's autonomous nervous system. The device detonates explosives hidden in their body in the event that the agent is terminated (if knocked unconscious, the agent won't detonate and you'll be able to loot the body). This eliminates any evidence of the agent and may damage his killer as well. MiBs are also expected to commit suicide in case of being unable to complete their mission objectives.
While not as dangerous as the MJ12 Commando (who wield rocket tubes and heavy caliber weapons), the Men in Black make up for it with sheer resilience. They have 400HP in all body locations which means from conventional weapons, only the sniper rifle, sawed-off shotgun and mini-crossbow loaded with normal darts can kill them with a single shot - and it must be a headshot on the Master skill level.
The Dragon's Tooth Sword can kill them in one stealth hit, but they explode on death and you won't be able to escape the blast without using the Speed Enhancement. It is worth noting that many MIBs (and WIBs) themselves are equipped with a copy of the nanotech blade, and can be extremely dangerous at close range.
Perhaps the best way of dealing with them is using the Riot Prod, as it knocks them out which means they don't explode, letting you loot their bodies. You will need to be Advanced in Low-tech weapons skill to knock them down in one stealth hit to the back, however. Alternatively, you can first hit them with the prod, and while they're shocked switch to the baton and hit them again. This will allow you to silently knock them out on Trained.
Women in Black have 50HP less than Men in Black, so they can be instantly knocked out with the prod at Trained, and if you use the baton trick - at Untrained.
MIBs and WIBs will not run away, no matter how injured. However, lighting them on fire will cause them to panic like any other NPC. Strangely enough, if left alone they will stop burning before the fire actually kills them.
MiBs and WiBs are often appointed commanders of MJ12 installations and strike teams. Most notable of them include: