The relations between the cytotoxic adherent cells (CAC) and the peroxidase positive adherent cells (PPAC) which appeared after i.p. injection of Brucella abortus were investigated. A kinetic study showed that PPAC preceeded CAC. Twenty-four hours after bacterial injection, almost all adherent cells stained for peroxidase and no cytotoxic activity could be recorded. Cells sampled at this moment acquired cytotoxic properties by cultivation in LPS containing medium. The treatment of the bacterial organisms by HCl or chloroform: methanol (C:M) suppressed their ability of inducing CAC but not that of inducing LPS-activatable PPAC. The substances extracted by C:M could activate PPAC into CAC in vitro. The data presented suggest that CAC differentiate from PPAC, that the recruitment of PPAC is not always followed by their transformation into CAC and that different components of the bacterial organisms might be necessary to achieve the steps of cell recruitment and maturation.