In the study complex organic mixtures that were extracts of particulate emissions from 3 heating sources were tested for the presence of agents that induce preneoplastic transformation of rat tracheal epithelial (RTE) cells in an in vitro clonal assay. The samples were derived from homes in Xuan Wei County, China where extremely high incidences of lung cancer are found that are 4-fold in excess of the national average. In some communes, the cancer mortality rates are as high as 19 to 30 times the national average rate. This is postulated to result from inhalation exposures produced by the routine burning of a 'smokey' coal in an open indoor pit as the sole source of heat and cooking fuel under unvented conditions. Dichloromethane extracts of particulate samples (<10 micrometers in size) of 'smokey' coal obtained from the respirable air in homes burning this fuel were tested in the RTE cell transformation assay. In addition, the response was compared to that obtained from particulate emissions of 2 other fuel types used in communes in China that have low associated lung cancer incidences: communes that use 'smokeless' coal, and wood as fuels.