A method for genotoxic testing of gaseous compounds or vapors using human lymphocytes was investigated. Heparinized human peripheral lymphocytes were prepared from healthy unrelated nonsmokers. The entire culture contents were placed into about 10 inches of dialysis tubing with a molecular weight cutoff of 12,000. The tubing was then placed in a 100 milliliter (ml) flask with a fritted bubbler containing 20ml of cultured medium. A small evaporation chamber containing the test compound was inserted in the air line into the bubbler and bubbled into the flask. After exposure, the tubing was removed from the flask, the cells were washed, resuspended in media and grown for the remainder of the 75 hour culture period. Cells were then scored for sister chromatid exchanges (SCEs). To test the method, cells were exposed to 245 or 418 parts per million ethylene- dibromide (106934) (EDB) gas for 0 to 160 minutes or to air only as a control. Air alone induced a slight, but significant increase in SCEs of 17 percent after 48 minutes exposure. In comparison, EDB produced an increase of 179 percent at 48 minutes. The increase in SCEs showed a clear response to increased time of EDB exposure, but was not significantly affected by the increase in EDB concentration. For one donor, SCEs/25 cells increased with time from 230 with no EDB treatment, to 265 at 5 minutes, 310 at 16 minutes, 379 at 48 minutes, and 449 at 160 minutes exposure. Cell cycle delay was apparent in each EDB exposure. The authors conclude that this procedure may be useful for testing gaseous or volatile compounds. Its use for in-situ monitoring of genotoxic agents in occupational settings is suggested. Alternative cell systems and biological end points might also be used. EDB induces high frequencies of SCEs in cultured human lymphocytes and may pose potential genetic, carcinogenic, and reproductive hazards to exposed workers.