induces invasive activity. Confluent cultures were serum-starved and treated with serum-free medium at pH 7.4 or pH 6.8 for 18 h. The conditioned medium was collected and the cells were harvested by trypsinization. The cells were subsequently incubated with 10% FBS for 30 min to inhibit trypsin activity, washed twice with PBS(−) and resuspended in their own conditioned medium. The chemoattractant was 20% FBS. A. Phase contrast micrograph. B. Relative numbers of invasive cells compared with LLCm1 cells at pH 7.4. Representative results were shown from three independent experiments. *P < 0.05; ***P < 0.001.