Disease-specific enhanced imaging through a targeted agent promises to improve the specificity of medical ultrasound. Nanoparticles may provide unique advantages for targeted ultrasound imaging due to their novel physical and surface properties. In this study, we examined a nanoparticle agent developed from a biodegradable polymer, polylactic acid (PLA). The nanoparticles (mean diameter = 250 nm) were surface conjugated to an anti-Her2 antibody (i.e., Herceptin) for specific binding to breast cancer cells that overexpress Her2 receptors. We examined the targeting specificity and the resultant ultrasound enhancement in Her2-positive and negative cells. Flow cytometry and confocal imaging were used to assess the nanoparticle-cell binding. Her2-positive cells demonstrated substantial staining after incubation with nanoparticle/antibody conjugates, while minimal staining was found in Her2-negative cells, indicating receptor-specific binding of the conjugated PLA nanoparticles. In high-resolution ultrasound B-mode images, the average gray scale of the Her2-positive cells was consistently and significantly higher after nanoparticle treatment (133 ± 4 in treated cells versus 109 ± 4 in control, p < 0.001, n = 5), while no difference was detected in the cells that did not overexpress the receptors (117 ± 3 in treated cells versus 118 ± 5 in control). In conclusion, the feasibility of using targeted nanoparticles to enhance ultrasonic images was demonstrated in vitro. This may be a promising approach to target cancer biomarkers for site-specific ultrasound imaging.