To investigate the biocompatibility of human gastric carcinoma cells (SGC-7901) with organic two-photon nanoparticles (NPs). Different concentrations of NPs were incubated with SGC-7901 cells for different times. The levels of cell apoptosis, reactive oxygen species (ROS), intracellular calcium, and mitochondrial membrane potential (MMP) were measured by staining the SGC-7901 cells with Annexin V-FITC/PI, 2',7'-dichlorofluorescin diacetate, Fluo-3 AM, and Rhodamine 123, followed by the flow cytometry assay. NPs at < 4 A mu g/ml, did not have any significant effect on apoptosis, necrosis, generation of ROS, increase of intracellular Ca2+ concentration or decrease of MMP in SGC-7901 cells, but > 4 A mu g/ml had a major effects on all the above mentioned parameters. 2,5,2',5'-Tetra(4-N,N-diphenylamine styryl) biphenyl NPs can be used at an appropriate concentration as a safe drug carrier or imaging marker and may serve as an effective tool for developing a photodynamic cancer therapy.