Costimulation of thymidine incorporation in BCR-stimulated B cells by BAFF and APRIL is mediated by both BAFF-R and TACI. TACI responds to oligomerized ligands only. Purified B splenocytes of various genotypes were cultured under BCR-stimulating conditions, with or without anti–BAFF-R B9C11, and with the indicated concentrations of BAFF and APRIL. After 48 hours, cells were pulsed for 16 hours with thymidine, harvested, and counted. Points represent mean plus or minus SD of triplicate cultures. Note that B9C11 abolishes BAFF-mediated effects through BAFF-R. At the concentration used, B9C11 totally blocked BAFF A134 at concentrations up to 10 ng/mL, but only partially at higher concentrations.