What does Lennie not have in his pocket?

In chapter one of Steinbeck's novella, the reader meets the main characters, George and Lennie, who are on their way to a farm job in the Salinas Valley. When they reach the Salinas River, the two men stop for the night. During their discussion, Lennie laments the fact that he seems to have lost his "work card." He searches his pockets without luck, telling George, "I ain't got mine. I musta lost it . . . I thought I put it in my side pocket." George then reveals that he has Lennie's card, mostly because he doesn't trust the big man to keep it safe. He says,

You never had none, you crazy bastard. I got both of 'em here. Think I'd let you carry your own work card?

Since George and Lennie are itinerant laborers they found work through agencies such as "Murray and Ready's" in Salinas who issued them cards to be presented to employers that used such agencies to supply their work force, especially during harvest time.
Instead of his work card, Lennie is carrying a dead mouse in his pocket. It seems the big man is trouble for anything soft that he wants to touch. The dead mouse episode foreshadows later events involving a puppy and Curley's wife.