A method using sister-chromatid exchanges (SCEs) for genotoxic testing of gaseous compounds is described. Human peripheral lymphocyte cultures previously stimulated with phytohemagglutinin were placed in sterile dialysis tubing and then put in an enclosed flask containing additional culture media. Air, with or without ethylene dibromide (EDB), was bubbled through the flask for up to 8 h. The cultures were harvested 75 h after culture initiation, and second-division cells were scored for induction of SCEs according to established procedures. The SCE frequency was approximately doubled in cultures treated with EDB. A similar experiment with air alone resulted in only slight increases in SCEs. The results indicate that this system is potentially useful for detecting genotoxicity of gases and vapors and may be useful for the detection of genotoxic agents in occupational settings.