Fletcher Traynor's father was the weapons dealer Harley Traynor, who had invented Heavy Mettle. He was killed in prison on the orders of his former employer Joe Silvermane. Fletcher, a student of the Massachusetts Academy, received letters from his father recounting what had happened and blamed the New Warriors for his death. Fletcher raided his father's store-house for an appropriate weapon to exact his revenge; he found a weapon that contained some green chemicals. Fletcher accidentally broke the trigger mechanism of the weapon and the cylinders containing the chemicals started to crack. Fletcher was doused with the chemicals and he mutated into a powerful, acidic creature. Fletcher dubbed himself Biohazard and the oozing mass of acid called out the New Warriors at King's Plaza Mall in Brooklyn.
Biohazard’s body has been transformed into viscous acidic toxic waste. He can expand in height and mass and project acidic slime geyser’s. He has a degree of invulnerability due to his semi-amorphous body, physical attacks pass through him. After alterations by Mr. Hyde he now constantly emits radiation and can split into smaller beings.