Research was conducted to analyze the physical and chemical properties of ashes from incinerated municipal sludge and of corresponding dewatered sludge. Samples were gathered from 10 wastewater treatment plants ranging in size from 0.22 to 27.1 cu/sec. These samples were subjected to a series of physical, chemical, and biological tests, including batch and chemical extractions and the Ames Salmonella assay. Results showed that the addition of supplemental chemicals during wastewater or sludge treatment strongly influenced the leachate properties and composition of both dewatered sludge and ash samples. In the absence of inorganic chemical addition, incineration acted generally to decrease trace metal availability. The behavior of arsenic during the various leaching tests was the opposite of that for metal cations with respect to pH. Incinerated sludges demonstrated weak mutagenic activity in the Ames assay. Four of the 10 ash samples gave positive results, generally in the presence of activating mammalian enzymes. In addition, one dewatered sludge sample also gave positive results . Both frameshift and base-pair mutagens were implicated.