Activation of CD27⁻ B cells with CpG DNA is a function of cell-cell contact conditions. CD27⁻ and CD27⁺ B cells were labeled with CFSE and activated with CpG DNA₂₀₀₆. After 4 days, proliferation was analyzed by flow cytometry. Activation and proliferation of CD27⁻ naive B cells required an increase in cell density to 1 × 10⁵ cells per well for maximal proliferation to occur. In contrast, CD27⁺ B cells proliferated following CpG DNA at all cell concentrations and densities, although higher cell densities led to an increase in proliferation. IL-10 improved proliferation of both CD27⁺ and CD27⁻ cells, while the combination of IL-2, IL-10, and IL-15 and cell-cell contact gave maximal proliferation. Results are representative of 15 independent experiments from 9 different blood donors.