The Man-Bat was a creature resulting from crossing of human and bat DNA.
Developed by Dr. Kirk Langstrom, the Man-Bat serum was a horrifying concoction that altered human genetic structure, and warped the user to become a vicious, feral half-human, half-bat creature—the Man-Bat. During the mutation, those affected would lose most of their mind and give in to their animalistic impulses, save in the direst of emergencies such as when a strong enough mental shock was applied.
Also, during the transformation, they would gain greater stamina and the ability to fly. Dr. Langstrom's father-in-law Dr. March of the Gotham Zoo refined serum and perfected it so the new one would also increase the sensitivity of the ear to include echolocation, an ability only the second Man-Bat demonstrated. The Man-Bat was the first successful example of splicing, and was based upon a serum created originally by Emile Dorian and eventually modified by Langstrom towards bat-like mutative properties.
The first serum was taken by the aforementioned Langstrom, who immediately fell under its influence and felt the beast's mind coexisting with his own, continually demanding the chemicals necessary to make the transformation permanent. Under the beast's influence, Langstrom continually raided chemical laboratories to acquire these substances. Thus, he came under conflict with Batman and was ultimately knocked unconscious. Batman took the defeated Man-Bat back to the Batcave, where Batman and Alfred worked on a means to cleanse the bat DNA and harmful chemicals. This apparently worked, as Batman returned a reverted Kirk Langstrom to his wife Francine.
Some time later, however, Langstrom suspected he had unconsciously again taken the Man-Bat persona, as he suffered nightmares and his fears were fueled by news reports that a bat-like creature had been terrorizing the port in order to help itself to food. This gained the attention of Batman, who confronted Langstrom. Langstrom denied resuming the bat experiments and chalked it up to the possibility of only a partial cure. Batman ran a DNA test to confirm, but before he could return with the results, Francine left her husband, claiming a return of her husband as the Man-Bat was too much for her. Batman later returned saying that Kirk Langstrom was indeed free of bat DNA, thus a new Man-Bat was about. The guilty culprit turned out to be Dr. March, who covertly continued the bat experiments as he was still obsessed with the concept that humanity could not survive an evolutionary upheaval.
Langstrom attempted and succeeded in reasoning with his wife aboard an airplane leaving Gotham before she left him indefinitely, but shortly after, she succumbed to the mutagen herself after she had accidentally absorbed it through a piece of cut glass in an accident in Dr. March's laboratory. Turning into the She-Bat, she started a rampage through the plane, escaping with her unconscious husband. Eventually, Batman confronted her atop one of the bridges surrounding Gotham. Injecting her with the cure, she was returned to normal.
Years later, Terry McGinnis was forcibly spliced by Abel Cuvier, who used a massive overdose of vampire bat serum to turn him into a feral creature similar to the original Man-Bat. As he began to transform, he returned to the Batcave to find help, but lost control and attacked his mentor, Bruce Wayne. Fortunately, after a brief struggle, Bruce was able to reverse the process using an antidote.
Anyone in the form of a Man-Bat had abilities similar to those of a bat, including flight, echolocation and enhanced hearing and strength.