In this study we investigated the cytotoxic activity mediated by antibody dependent cells (K cell) in C57BL/6 mice experimentally infected with Brucella abortus. The involvement of the K cells in experimental infection from Brucella was studied by evaluating the bacteriolysis in the presence of specific antibodies. After 3 h of incubation, approximately 30 +/- 3% of the labeled Brucellae undergo lysis, whereas the presence of lymphocytic populations alone, or of a specific decomplemented serum, determines a lysis of 5 +/- 3%. Cytotoxic antibody dependent activity of purified lymphomonocytic populations (ADCC) evaluated against cells labeled with 51Cr sensitized with specific antibodies obtained from mice is considerably increased in mice experimentally infected with Brucellae with respect to that present in non-infected mice. The increased activity begins to manifest itself 5 to 7 days after inoculation of the Brucellae reaching a maximum on the 10th day. The same results were obtained in the lymphocytic populations prepared from normal animals and those infected by Brucellae searching for K cells with the ability to form rosettes (EA-RFC).