To study the effect of particulate extracts (PE) collected from a heavy traffic road in Lanzhou City, on MRC-5 cell apoptosis, and to explore the toxicity action of PE and its mechanism. Cultured MRC-5 cells were incubated in the extracts of different concentrations. Inhibition of proliferation was measured with a colorimetric 3-[4,5-dimethyl thiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Morphological assessment of apoptosis was performed with fluorescence microscopy and electronic microscopy. Extracted DNA from the cells was electrophoresed on agarose gel in order to observe DNA fragmentation. The amount of apoptotic cells was measured by flow cytometry. The results indicated that exposure of exponentially growing MRC-5 cells exposed to PE 5-160 microg l(-1) for 24-96 h resulted in dose- and time-dependent reduction of survival of MRC-5 cells. After treatment with PE, markedly morphological changes of MRC-5 cells including "apoptotic bodies", were observed with a fluorescence microscope. Agarose gel electrophoresis of DNA from the cells treated with PE for 48 and 72 h revealed a "ladder" pattern. PE induced apoptosis in low doses but necrosis in high doses. Apoptotic rates were 12.95, 17.40 and 29.80% after treatment with PE 5, 10, and 20 microg l(-1), respectively. A typical sub-diploid apoptosis peak was demonstrated in MRC-5 cells treated with PE. A significant dose-effect response and time-effect correlation could be found between apoptosis rates and PE. All results confirmed that the PE could induce and accelerate apoptosis in low doses but necrosis in high doses.