BAFF-induced B-cell proliferation by neonatal cord blood–derived B cells is decreased compared with adult B cells. Naive B cells were stimulated with anti-IgM (10 μg/mL) in the presence or absence of BAFF (100 ng/mL) and tested for proliferation. For the blocking assay the TACI-Ig fusion protein was used at 2 μg/mL. Proliferation was measured by ³H-thymidine [1 μCi (0.037 MBq) per well] incorporation of B cells during the last 18 hours of a total of 96 hours. Data are shown as mean (± SEM) of 10 independent experiments using 10 independent adult (AB), cord (CB), or preterm cord (PreCB) blood samples with each assay performed in triplicate. Statistically significant differences are shown. *P < .001; **P ≤ .005.