Flow cytometry of peripheral blood samples can detect low levels of circulating lymphoma cells. A previous study has demonstrated that detection of lymphoma cells by peripheral blood flow cytometry has a good correlation with bone marrow involvement. This study will seek to find if this correlation is evident in a larger, statistically significant patient cohort. Depending on the study results, invasive and painful bone marrow procedures could potentially be reduced or eliminated in patients with certain slow growing lymphomas. Patients diagnosed with follicular lymphoma, mantle cell lymphoma or marginal zone lymphoma and being treated at the Royal Marsden NHS Foundation Trust will be eligible for this study. Participation in the study will require one peripheral blood sample to be taken before the patient's routine bone marrow biopsy. The study is expected to last between 2 - 3 years for data collection of 108 samples.