"Monster Cells" (怪人細胞, Kaijin Saibō) is the thirteenth volume of the One-Punch Man manga series.
A group of A-Class superheroes gather to fight a large octopus monster, but Flashy Flash arrives and pokes out all of the octopus' suckers where there are eyes. The octopus is angered, but then Tornado arrives and crushes it with her psychic powers into a single octopus ball. Atomic Samurai meets with a council of swordmasters on what to do about Garou. One of its members, Haragiri, tells everyone to ingest monster cells in order to become strong like the monsters. Seeing that Haragiri has already become such a monster, Atomic Samurai immediately destroys Haragiri by slashing him into pieces. Saitama advances to the finals and faces Suiryu, who tells him he would like the match to be fun. After some near-miss shots, he starts fighting for real until he knocks off Saitama's wig. Saitama is disqualified for wearing a disguise. Suiryu still wants to fight, but Saitama doesn't budge and hip checks him out of the arena. The monsters arrive, rounding up all the injured participants. A fierce-looking lead monster Gouketsu announces his plan to convert the fighters into monsters. The bonus chapter focuses on Amai Mask being attacked by monsters during one of his concerts.
| 骨 ほね|