Carbon black is frequently confused with soot. In order to clarify the distinction between these materials, we have examined 6 carbon blacks and 18 samples of soot. Carbon black is composed of turbostratic colloidal aggregates which we call aciniform carbon (AC). Chimney soots from domestic wood or coal fires contain very little AC, while in diesel soots the solid particulates are essentially all AC. All soots examined contain much more extractable organic material and/or ash than does carbon black. Ames Salmonella assays were carried out on solvent extracts. The results were calculated as net revertants per mg of the original soot or carbon black. On this basis, the majority of the soot extracts assayed were at a much higher level of mutagenicity than any of the carbon black extracts. For several reasons, even greater differences in mutagen availability would be expected if the unextracted materials were compared in vivo.