Monster Blood is a green substance that consumes things, and anything that consumes it will grow huge.
Monster Blood first appeared in the Goosebumps book Monster Blood where it is opened by Evan and Andy, soon afterwords Evan's dog Trigger eats some. Then it appeared in Monster Blood II when Cuddles the Hamster eats some. It was in Monster Blood III and that time Evan ate some.
Finally for the last book in the original Goosebumps series book Monster Blood IV where it is blue, whenever water hits it it will multiply, get eviler and grow fangs. The blue Monster Blood also makes a person multiply when someone eats it.
Then, in the third Goosebumps HorrorLand book, "Monster Blood For Breakfast!", the Monster Blood was a "12 hour Trial Strength" version, and it came in a plastic egg and made a new person, Matt Daniels, grow about twelve feet tall.
It then followed him to HorrorLand where it sucked up Sheena Deep and a horror named Byron who saved Matt and Sheena's brother, Billy. It also appeared in the television episodes Monster Blood and More Monster Blood, where airline food makes it shrink. It will only stop growing when it is put back in its jar.