BACKGROUND We aimed to evaluate the potential for ultrasound (US) visible biodegradable nanoshells (NS) as an alternative to wire-guided localization for nonpalpable tumors in vivo. METHODS VX2 tumor was injected in bilateral thighs of 22 New Zealand rabbits and after 5 to 10 days, 1 tumor was marked with a wire as a control and the contralateral tumor was injected with 1 mL of 500 nm gas-filled silica NS under Doppler US. Tumors were excised after 24 hours. Chi-square was used for significance, P = .05. RESULTS One rabbit was excluded on postoperative day 1 due to equipment failure, no ill effects were observed from the NS. The NS were used to localize and resect 100% of marked tissue, 4/21 wires were displaced (P < .05). CONCLUSIONS We have shown that preoperatively injected US visible silica NS can be successfully used to mark nonpalpable tumors in vivo more consistently than WL.