A reverse transcriptase real-time polymerase chain reaction was developed to estimate the expression of the gamma-interferon (IFN-gamma) and interleukin-4 (IL-4) in cattle vaccinated with Brucella abortus RB51. Peripheral blood mononuclear cells of heifers vaccinated with B. abortus RB51 were stimulated in vitro with the same antigen and with Concanavalin A. The data obtained (presence of IFN-gamma expression and absence of IL-4 expression) confirmed the cell-mediated immune response to strain RB51 antigen. Furthermore, the expression of these two cytokines was quantified and the results showed values of IFN-gamma expression to be significantly higher in vaccinated than non-vaccinated animals.