The pet monkey of Kryptonite Man, who is really Lex Luthor. Kryptonite Monkey is originally just a lab monkey until Lex Luthor tests his Kryptonite-based serum, which he creates by dissolving a Kryptonite meteor, on the monkey. Once he tests the serum on the lab monkey, the monkey begins to glow with a green color and emanates Kryptonite radiation from his body. Ultimately, Kryptonite Monkey escapes Luthor's lab and goes to the Metropolis Zoo where Clark Kent, who is secretly Superman, is on assignment. When Clark Kent spots Kryptonite Monkey he thinks to himself:
Great Guns! Kryptonite radiations ... from that glowing monkey... are weakening me!
Clark Kent manages to spot a "leaden drainpipe" and decides to uses his super-breath to blow a banana from a nearby boy's hand toward the drainpipe. Clark's plan works as:
"he (Kryptonite Monkey) dived into the leaden drainpipe for the banana! Kryptonite radiations can't penetrate lead!"
Superman recovers and flies away before Kryptonite Monkey can come out from the "leaden drainpipe" (Act No. 249, Feb 1959: "The Kryptonite Man!").