Tomcat is a quest given by Butch Billington. Upon arriving at Tom 'n Butch's hideout Joe hears Butch Billington crying out of help. He tells Joe that Tom Catt has been bitten and asks if he has any Green Strain Antigen.
Even if you are carrying some antigen at the time it's best to refuse to give it to him or else you'll miss out on this quest and you'll also miss some extra Approval and Respect for completing the quest. Butch will give you the location of an abandoned police car north of Santa Maddalena where there is a secret stash of Green Strain Antigen. After you manage to find some antigen you can either sell it to Butch for 50g or give it to him for free.