Incubation of Brucella abortus (field strain) infected and strain 19 vaccinated bovine peripheral blood lymphocytes with B. abortus antigen and levamisole caused a consistently significant increase in [3H] thymidine uptake when compared to cultures without levamisole. Levamisole did not potentiate B. abortus-induced blastogenic response of lymphocytes from non-exposed cattle. A dose response study showed that 10 μg/culture induced maximum potentiation of B. abortus-induced lymphocyte stimulation. Using the 10 μg/well concentration of levamisole, further studies were conducted to determine the net potentiation of the blastogenic responses in lymphocytes from B. abortus (field strain) infected cattle, B. abortus strain 19 vaccinated but nonresponsive and non-exposed cattle. Levamisole significantly potentiated the B. abortus-induced lymphocyte blastogenesis in lymphocytes from unresponsive cattle.