The biological activities of whole-cell pertussis vaccine and a new acellular preparation were compared. It was observed that both preparations were similarly effective in inducing protection in mice against intracerebral challenge with B. pertussis, however, the acellular preparation was approximately 200-fold more potent. A close relationship between the degree of pertussis toxin purification and the increase in potency was observed. Neither preparation affected the weight gain of the animals within the doses employed. As expected, both preparations had histamine-sensitizing activity and leukocyte-promoting activity. It is suggested that this preparation, obtained using very simple methodology, can be considered as a potentially useful acellular pertussis vaccine.