We are evaluating PCT imaging in conjunction with NIR dye labeled Herceptin antibody for noninvasive assessment of HER2 expression in tumors. Herceptin was labeled with Alexa Fluor-750 amine reactive dye for characterization of photoacoustic and fluorescence signals. Measurements were performed in solution and after incubation in cultured cell lines that were positive or negative in expression of HER2. The dye to antibody ratio was controlled to achieve a broad range of degree of labeling (DOL = 2 to 15). Photoacoustic signal intensity of Herceptin-dye conjugates in solution increased with increases over the entire DOL range studied. In contrast, fluorescence exhibited significant quenching for higher DOL. In vitro PCT imaging of the labeled HER2 (+) and HER2 (-) cells revealed the targeting specificity of the NIR dye labeled Herceptin. In HER2 (+) cells lines, photoacoustic signal intensity gradually increased with increasing DOL and with increasing number of cells. These results demonstrate that PCT-based measurement of HER2 receptor binding using NIR dye labeled Herceptin is feasible. The absence of a quenching effect with increased DOL advantages this method over traditional methods based on fluorescence measurement.