Gold nanocages with an average edge length of 65 +/- 7 nm and a strong absorption peak at 800 nm were conjugated with monoclonal antibodies (anti-HER2) to target breast cancer cells (SK-BR-3) through the epidermal growth factor receptor (in this case, HER2), which is overexpressed on the surfaces of the cells. Both the number of immuno Au nanocages immobilized per cell and the photothermal therapeutic effect were quantified using flow cytometry. The targeted cells were irradiated with a pulsed near-infrared laser, and by varying the power density, the duration of laser exposure, and the time of response after irradiation, we were able to optimize the treatment conditions to achieve effective destruction of the cancer cells. We found that cells targeted with the immuno Au nanocages responded immediately to laser irradiation and that the cellular damage was irreversible at power densities greater than 1.6 W/cm(2). The percentage of dead cells increased with increasing exposure time up to 5 min and then became steady. By quantifying the photothermal effect of immuno Au nanocages, critical information with regards to both the optimal dosage of nanocages and parameters of the laser irradiation has been garnered and will be applied to future in vivo studies.