A Study of the Kinetics of Lymphoid Cells in Patients With Monoclonal B-cell Lymphocytosis (MBL), Chronic Lymphocytic Leukemia (CLL) or Small Lymphocytic Lymphoma (SLL), Mantle Cell Lymphoma (MCL) and Healthy Volunteers

Background: * Chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), and mantle cell lymphoma (MCL) are types of cancers in which there are too many abnormal lymphocytes (a type of white blood cell). Monoclonal B-cell lymphocytosis (MBL) is a condition in which the individual has a larger than normal number of lymphocytes. Individuals with CLL, SLL, MBL, and MCL may survive for many years without the need for treatment, but there is an apparent correlation between cell birth rates and disease activity. By studying the birth and death rates of lymphocytes, researchers hope to identify individuals who are at risk for worsening disease. * Heavy water is similar in structure to regular water, but it has two deuterium atoms instead of two hydrogen atoms. Deuterium has one more neutron than hydrogen, which is what makes heavy water heavy. Heavy water is not radioactive, looks and tastes like regular water, and has no known harmful effects at research-level doses. When a small amount of heavy water is consumed daily, newly produced blood cells are labeled (tagged), which allows researchers to track cell growth and to measure the birth and death rates of CLL, SLL, MBL, MCL or normal lymphocytes. Objectives: \- To study the birth and death rates of lymphocytes from individuals with MBL, CLL/SLL, and MCL, compared with lymphocytes from healthy volunteers. Eligibility: * Individuals at least 18 years of age who have been diagnosed with MBL, CLL, SLL, or MCL, but who have not been taking certain agents (Viagra, Levitra, Cialis, or other PDE-inhibitors, prednisone, cyclosporin-A, rapamycin, or other immunosuppressive agents, more than 2 cups of green tea daily, or Celebrex) for 4 weeks prior to enrollment in the study. * Healthy volunteers at least 18 years of age, but who have not been taking certain agents (Viagra, Levitra, Cialis, or other PDE-inhibitors, prednisone, cyclosporin-A, rapamycin, or other immunosuppressive agents, more than 2 cups of green tea daily, or Celebrex)for 4 weeks prior to enrollment in the study. Design: * Participants will be screened with a medical history, physical examination, and initial blood tests. Other tests may be administered to the individuals with cancer, as required by the study researchers. * All participants will drink regular doses of heavy water daily for a total of 4 weeks (labeling period). There is an optional 6-month follow-up or wash-out period during which no additional heavy water will be consumed. * Blood samples will be collected weekly during the labeling period, and a bone marrow biopsy will be obtained where possible. Individuals with cancer may also have a lymph node biopsy during this part of the study. * Additional blood samples may be collected during the optional wash-out phase of the study to determine the rate at which cancer cells disappear. * Treatment is not provided as part of this protocol.