This report deals with some ultrastructural and functional changes which were observed in human monocytes from peripheral blood in patients treated with Brucella vaccine during the remittance phase of an acute B. melitensis infection. Monocytes from these patients showed increased “in vitro” phagocytic and bactericidal activities and displayed morphological attributes which are considered characteristic of the so-called macrophage activation, a functional modification observed solely in animals in association with cell-mediated immunity states. The authors discuss their findings in relation to previous results obtained in human tuberculin hypersensitivity and put forward the hypothesis that the observed modifications in morphology and function of monocytes might be obtained through release by sensitized lymphocytes of lymphokine factors acting on mononuclear phagocytes.