Prednisolone, given orally or intraperitoneally before challenge, protected mice against the lethal effect of a crude cell extract of Bordetella pertussis containing heat-labile toxin (HLT) as the major toxic component. Prednisolone did not diminish the lethal toxicity of heated B. pertussis cell suspensions containing pertussis toxin and endotoxin but devoid of HLT. This suggests that the protective effect of the steroid was directed against the HLT. When live bacteria were injected intraperitoneally, prednisolone showed a protective effect against the initial toxaemia. By day 7, however, the protection was no longer evident and the steroid promoted the survival of the organisms within the peritoneal cavity. These findings are discussed in the light of reports of the beneficial effects of corticosteroids in the treatment of whooping cough and in relation to a possible role for HLT in the pathogenesis of the disease.