This study was designed to examine suspended air particulates from the Houston atmosphere, Airborne particulates were collected using either a hi-vol sampler (one stage from 0.01 to 25 micrometer) or an Anderson Cascade Impactor, the five stages of which roughly resemble the human respiratory tract. After organic extraction, the Ames assay was used to determine the mutagenic content of extracts, and the ability to induce prophage was assessed. Also DNA-repair-deficient cells were employed to see if the extracts caused DNA damage and what portion of the premutational lesions was repaired in normal cells. Results indicate that extracts of particulates from Houston air cause a significant number of mutations in bacteria and that the highest frequency of reversions is associated with the smallest particulates. An excision repair system is operative in bacteria which is able to assuage damage done to DNA by these extracts. The extracts did not cause prophage induction.