Some physical, chemical, and immunological properties of filamentous appendages and the exosporium on the spores of Bacillus cereus were examined for the purpose of elucidating the origin of filamentous appendages. The main components of both filamentous appendages and the exosporium were protein and their amino acid compositions were similar in point of a high content of glycine, alanine, threonine, valine, and acidic amino acids and a low content of basic and sulphur-containing amino acids. Treatment with 1 N NaOH at 50 C solubilized the isolated appendages completely and the isolated exosporia partially. In both preparations the solubilized proteins consisted of highly acidic monomeric subunits with molecular weights between 2,000 and 5,000. Treatment of the spores with 2% 2-mercaptoethanol at 37 C resulted in the isolation of long filamentous appendages without segmentation. When the spores were treated with 10% 2-mercaptoethanol, there was partial destruction of the exosporium as well as detachment of the filamentous appendages. There was a common antigenic component in the exosporium and the tips of the filamentous appendages. Five strains of B. cereus having a common appendage antigen also had a common exosporium antigen, whereas six other strains had neither a common appendage antigen nor a common exosporium antigen. From these facts it was concluded that the filamentous appendages arose from the exosporium.