For this study, you will receive rituximab by vein followed by a dose of 111In Zevalin by vein (over 15 minutes). 111In Zevalin is different from the study drug (90Y Zevalin). 111In Zevalin has a different type of radioactive particle attached to it. This particle does not kill lymphoma cells, but it can be "seen" inside the body using a special camera (like an x-ray). 111In Zevalin is being used to predict how fast the study drug will travel in the body and how long the drug stays in the body. Doctors need to be able to see how much of the drug goes to the tumor and how much goes to normal organs to determine if it is safe to give 90Y Zevalin on an outpatient basis. A scan will be done as soon as 111In Zevalin is given and about 2-24 hours later. Scans will also be done 2-3 days later. If the radiation in the 111In Zevalin is not a threat to normal organs and bone marrow, you may receive 90Y Zevalin. Seven (7) days after the 111In Zevalin injection, you will receive a second dose of rituximab followed by a dose of 90Y Zevalin by vein (over 15 minutes).