If you are found to be eligible to take part in this study, you will be given diphenhydramine (Benadryl) by vein and you will take acetaminophen (Tylenol) by mouth before each dose of rituximab. This is done to help decrease the risk of developing side effects of rituximab. You will then receive 1 dose of rituximab by vein over about 4-6 hours on Day 1 of treatment. After treatment with rituximab, you will then be given 111 In Zevalin (this is a radioactive agent that binds to rituximab to help with imaging exams), by vein over about 10 minutes. This is so researchers can use a special camera to see where the drug is in your body.