Tell the prospective employer that you think it's too low. They may:
- Increase their offer
- Tell you that it's not negotiable
- Tell you to take a hike
Of the three possibilities, only one results in you being worse off then you would be if you don't ask, and it's also the least likely to occur. (After all, they want to hire you. Put yourself in their shoes: would you lose all interest in hiring someone simply because they asked for more money?)
Do be aware that your bargaining position is very weak (straight out of school, no competing offers?), but it is very much in your interest to ask anyway.