By means of anaerobic culture for 3-4 weeks a variant strain of Propionibacterium acnes was isolated from one lymph-node biopsy specimen, and from blood samples of five of twenty-three patients with early Kawasaki disease, but from only one of fifteen blood samples from patients after 8 days' illness. No anaerobe was isolated from sixty age-matched controls with various disorders, but the same bacillus with the same serotype was isolated from house-dust mites from six patients' homes. Patients had significantly higher serum agglutination titres to these strains than controls. The antigen moiety of P acnes was found in the patients' circulating immune complexes. Inoculation of animals caused various inflammatory lesions, particularly in the reticuloendothelial system, and coronary arteritis, myocarditis, and endocarditis in one of them, suggesting that the bacillus is pathogenic. The culture filtrate of this strain showed toxicity in tissue culture. This variant strain of P acnes may have a causative role in Kawasaki disease and house-dust mites a role as vectors.