Middle-aged and aged B cells show decreased ability to proliferate under stimulating conditions. SPC from young, middle-aged, and aged QM mice were harvested and cultured in triplicate with anti-CD40 mAb or LPS for 24 hours and incubated with [3H]-Tdr and incubated another 24 hours. CPM was measured as described in material and methods. Error bars are shown for triplicate samples. Middle-aged and aged B cells stimulated with LPS show a significant decrease in proliferative capacity (P = 0.03 and 0.005 respectively). Aged B cells stimulated with LPS show a significant decrease (P = 0.029) while middle-aged cells appear to be decrease are not significantly different (P = 0.09). Data represents 4 experiments. P values determined by student's T-test.