PI, a lipid A-free fraction (15) extracted from the cell wall of Brucella melitensis type I M.15 behaves as a non specific mitogen and a polyclonal B-cell activator (PBA) for human unsensitized lymphocytes from subjects with no previous history and no biological stigmata of brucellosis. Both effects may be evidenced, indifferently, in cultures of peripheral blood, tonsils or cord blood lymphocytes. Mitogenicity, estimated by the incorporation of tritiated thymidine is maximal on the 5th and 6th day of culture, for high cellular concentrations and for 10 micrograms/PI/well. PBA activity is estimated by the incidence of Prot. A reverse PFC and by a laser nephelometric measure of immunoglobulins in culture supernatants. The use of ET-rosetting for the preparation of purified and/or reconstituted populations demonstrates the T-dependence of the B cell activating effect of PI. On the other hand adherent cell depletion on Sephadex G 10 columns does not significantly impair these effects. Important individual differences are observed and suggest a genetic control of the level of B responses to this new bacterial PBA. These results are discussed in terms of a comparison of PI with other human PBA and of the interactions between non specific and specific responses of lymphocytes from vaccinated subjects or brucellosis patients.