Study of AMD3100 (Plerixafor) and Rituximab in Patients With Chronic Lymphocytic Leukemia or Small Lymphocytic Lymphoma

Inclusion Criteria: * Females of child bearing potential must agree to abstain from sexual activity or to use a medically approved contraceptive measure/regimen during and for 3 months after the treatment period or be surgically sterile. Males must agree to abstain from sexual activity or agree to utilize a medically approved contraception method during treatment and for 3 months after the treatment period or be surgically sterile. * Diagnosis of CLL or SLL, relapsed from at least one prior therapy. * CLL/SLL cells expressing CD20 documented during screening. * Eastern Cooperative Oncology Group (ECOG) performance status of ≤2. * Life expectancy of at least 12 weeks. * Serum creatinine ≤2.0 mg/dL. * Total bilirubin ≤2.0 mg/dL. * ALT (alanine aminotransferase) and AST (aspartate aminotransaminase) ≤2 times the upper limit of normal (ULN); for patients with liver involvement of CLL/SLL disease, this limit is increased to ≤5 times the ULN. * At the time of enrollment, patients must be \>4 weeks since major surgery, radiotherapy, chemotherapy (\>6 weeks for some chemotherapies), immunotherapy, biotherapy/targeted or investigational therapies and recovered from the toxicity of prior treatment to ≤ grade 1. Exclusion Criteria: * White Blood Cells (WBC) \>250 x 10\^9 cells/L. * Disease refractory to rituximab therapy- defined as a failure to respond to prior rituximab-containing regimen. * Women who are breastfeeding. * Active viral hepatitis. * Active infection or treatment with antimicrobial or antiviral therapy within 1 week of enrollment with the exception of prophylactic therapy. * History of prior allergic reaction to plerixafor or rituximab. * Significant lung disease. * Serious cardiac disease such as a history of sustained ventricular arrhythmia, uncontrolled and serious congestive heart failure (CHF), angina, acute coronary syndrome, or myocardial infarction within 6 months of enrollment or other significant medical or psychosocial conditions that warrants exclusion.