Advanced ovarian cancer is largely incurable, but initially it is frequently confined to the i.p. space. We explored i.p. radioimmunotherapy in a mouse model of human ovarian cancer. Use of a targeted actinium-225 (²²⁵Ac) in vivo generator of α particles exploits the extreme, selective cytotoxicity of α particles, while providing a feasible half-life to enable delivery to tumor. ²²⁵Ac chelated with 2-(p-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10 tetraacetic acid was conjugated to trastuzumab, an anti-HER-2/neu antibody. The radioimmunoconjugate was tested for immunoreactivity, internalization, and cytotoxicity using a human ovarian carcinoma cell line, SKOV3. ²²⁵Ac-labeled trastuzumab retained immunoreactivity (50–90%), rapidly internalized into cells (50% at 2 h), and had an ED₅₀ of 1.3 nCi/ml after 4 days of incubation in vitro. i.p. administered ²²⁵Ac- or ¹¹¹In-labeled trastuzumab behaved similarly with high tumor uptake [56–60% injected dose per gram (% ID/g) at 4 h, which increased to 65–70% ID/g at 24 h]. Tumor uptake was 3–5-fold higher than liver and spleen, the normal organs with the highest uptake. i.v. administration of ¹¹¹In-labeled trastuzumab produced slightly higher normal organ uptake compared with i.p.-administered ¹¹¹In-labeled trastuzumab. However, tumor uptake was low, 5%–26% ID/g. Therapy was examined with native trastuzumab and 220, 330, and 450 nCi of ²²⁵Ac-labeled trastuzumab or ²²⁵Ac-labeled control antibody at different dosing schedules. Therapy was initiated 9 days after tumor seeding. Groups of control mice and those administered native trastuzumab had median survivals of 33 and 37 or 44 days, respectively. Median survival was 52–126 days with ²²⁵Ac-labeled trastuzumab at various doses and schedules, and 48–64 days for ²²⁵Ac-labeled control the same schedules. Deaths from toxicity occurred with the highest activity levels. In conclusion, i.p. administration with a ²²⁵Ac-labeled internalizing anti-HER-2/neu antibody can extend survival significantly in a nude mouse model of human ovarian cancer at levels that produce no apparent gross toxicity.