Human B cells detect CpG motifs within microbial DNA via TLR9. Synthetic CpG oligodeoxynucleotides are currently being tested in clinical trials for the therapy of different types of B cell non-Hodgkin's lymphoma. However, there is only limited information on the CpG oligodeoxynucleotide sensitivity of primary malignant B cells of different non-Hodgkin's lymphoma entities. Here we found that most B-cell malignancies except plasmacytoma respond to CpG oligodeoxynucleotides by up-regulating expression of costimulatory and antigen-presenting molecules, by increasing expression of CD20, and by proliferation. In an in vitro analysis of 41 individual patient-derived primary tumor samples, B-cell chronic lymphocytic leukemia (B-CLL) and marginal zone lymphoma showed the strongest activation upon stimulation with CpG oligodeoxynucleotides. Small lymphocytic lymphoma, follicular lymphoma, mantle cell lymphoma, and large cell lymphoma showed an intermediate response. Consistent with CpG oligodeoxynucleotides sensitivity, TLR9 mRNA was present in B-CLL but absent in plasmacytoma. Although CpG oligodeoxynucleotides induced proliferation in all CpG oligodeoxynucleotide–sensitive types of B-cell malignancies, proliferation was weaker than in normal B cells and at least for B-CLL was followed by increased apoptosis. In conclusion, B-cell malignancies show significant differences in their responsiveness to CpG oligodeoxynucleotides. Focusing clinical studies on patients with highly CpG oligodeoxynucleotide–sensitive B-cell malignancies may improve the clinical outcome of such trials.