A murine encephalopathic syndrome can be induced by the administration of BSA and whole-cell pertussis vaccine. The present paper reports studies of the capacity of purified individual pertussis components to induce this effect. Pertussis toxin and endotoxin together with a highly immunogenic sensitizer protein were required to induce the effect. The strength of the antibody response to the sensitizer appeared to be more important than the H-2 type of the recipient in determining the susceptibility of different mouse strains. The relevance of this syndrome to the study of possible vaccine-induced encephalopathy in man is uncertain and requires further investigation.