Serial cytogenetic observations were made on a group of 273 military recruits who were being trained as welders at Aberdeen, Maryland. The trainees were being exposed to presumably increased levels of ozone in the course of their welding school experience, and it was the purpose of this study to determine whether or not ozone, at low to moderate doses, is capable of inducing chromosomal aberrations in peripheral blood lymphocytes. Previous exposures and the past medical experiences of the trainees were determined by questionnaire, enabling us to obtain a profile on the medical-social characteristics of the study sample. Each welder was to serve as his own control, having a blood sample drawn at the beginning of his twelve week training program, prior to ozone exposure, with two post-exposure bloods being obtained at six and twelve weeks after the start of the program. Ozone levels and the levels of the oxides of nitrogen were determined in the immediate area of the welding. The ozone levels were repeatedly found to be negligible, with the nitrogen oxides appearing to be the primary toxic agents involved. For those 165 subjects on whom two blood samples were obtained, and for those 86 on whom all three blood samples were obtained, no statistically significant increases in chromosomal aberrations were found.