The genotoxicity of coal dust extract nitrosated by NaNO2 was investigated because of an elevated incidence of gastric cancer in coal miners. Human peripheral lymphocytes were used to determine the frequency of sister chromatic exchanges (SCE) and chromosome aberrations. Chinese Hamster Ovary (CHO) cells were also used to measure SCEs. The mouse lymphoma forward mutation assay comprised the final in vitro system, while the micronucleus test on mouse bone marrow cells was used as an in vivo assay. The SCE frequency in human lymphocytes increased from 9.2 per cell in untreated cultures to 31.6 per cell in cultures treated with 10.0 microliter/ml of nitrosated coal dust extract (NCDE) (P less than .0001). Chromosome aberrations were tested during two phases of the cell cycle; the results indicate that NCDE causes an increase in aberrations in each cell phase tested. The SCE frequency in the CHO system increased from 9.0 per cell in untreated cultures to 18.0 per cell in cultures treated with 3.3 microliter NCDE per ml of medium (P less than .001). In the mouse lymphoma system, the background mutation frequency was tripled at 2.0 microliter NCDE per ml of medium. NCDE was not found to be active in the murine micronucleus assay. No consistent increase in genetic activity was observed with nonnitrosated coal dust extract alone or with NaNO2 alone. Metabolic activation greatly reduced or eliminated genetic activity. These results indicate that nitrosated coal dust extract is genetically active in in vitro mammalian systems without metabolic activation. These findings suggest that the nitrosation of ingested coal dust may be responsible for the elevated incidence of gastric cancer in coal miners.