A bio-molecular warhead was a specially-modified photon torpedo utilized as a delivery-system for Borg nanoprobes with which to combat Species 8472. Designed in 2374 by the crew of the USS Voyager in collaboration with the Borg Collective, the warheads contained a series of nanoprobes modified by Voyager's EMH to attack 8472 on a microscopic level, constituting a weapon which could not be replicated by the Borg, necessitating an alliance.
The crew constructed thirteen warheads using standard class-6 photon torpedoes, and one class-10 armed with a high-yield warhead. Entering fluidic space, Species 8472's native realm, Voyager engaged in combat with four bioships, successfully destroying them with the warheads. Voyager was subsequently pursued by 8472 back into normal space, where the Voyager crew utilized the high-yield warhead, destroying thirteen bioships, and prompting 8472 to retreat to fluidic space. (VOY: "Scorpion, Part II")