Bordetella pertussis(B.p.) induces blast transformation of human lymphocytes; whole killed B.p. are more efficient than extracts obtained by sonication. Similar responses were obtained with each of the four strains used in the Danish pertussis vaccine. B.p. with low amounts of Protective Antigen and Histamine‐Sensitizing Factor also induced lymphocyte transformation, but were less toxic to the lymphocytes at high concentrations. The supernatants of B.p. cultures were purified with respect to Lymphocytosis Promoting Factor; evidence is presented that these purified fractions possess T‐lymphocyte mitogenic activity. Lymphocytes from all normal humans were stimulated by B.p., including cells from cord blood. Cells from child‐bearing women, obtained immediately after delivery, showed a general depression of lymphocyte transformation including the response to B.p. Children with whooping cough had a lower lymphocyte response to B.p. than healthy children. A highly significant correlation was observed between the responses to B.p. and toE. coliin the adults and newborn examined. It is concluded that the major part of the lymphocyte transformation induced by B.p. is non‐s