The purpose of this study was to compare the therapeutic efficacy and biodistribution of the monoclonal antibody MX35 labeled with either 213 Bi or 211 At, both α-emitters, in an ovarian cancer model.One hundred female nude BALB/c (nu/nu) mice were inoculated intraperitoneally with human ovarian cancer cells (OVCAR-3). Two weeks later, 40 of these mice were injected intraperitoneally with ∼2.7 MBq of 213 Bi-MX35 (n=20) or ∼0.44 MBq of 211 At-MX35 (n=20). Four weeks after inoculation, 40 new OVCAR-3-inoculated mice were injected with the same activities of 213 Bi-MX35 (n=20) or 211 At-MX35 (n=20). Presence of tumors and ascites was investigated 8 weeks after therapy. Biodistributions of intraperitoneally injected 213 Bi-MX35 and 211 At-MX35 were studied in tumor-free nude BALB/c (nu/nu) mice (n=16).The animals injected with 213 Bi-MX35 or 211 At-MX35 2 weeks after cell inoculation had tumor-free fractions (TFFs) of 0.60 and 0.90, respectively. The untreated reference group had a TFF of 0.20. The groups treated with 213 Bi-MX35 or 211 At-MX35 4 weeks after inoculation both had TFFs of 0.25, and the reference animals all exhibited evidence of disease. The biodistributions of 213 Bi-MX35 and 211 At-MX35 were very similar to each other and displayed no alarming activity levels in the investigated organs.Micrometastatic growth of an ovarian cancer cell line was reduced in nude mice after treatment with 213 Bi-MX35or 211 At-MX35. Treatment with 211 At-MX35 provided a non-significantly better result for the chosen activity levels. The radiolabeled MX35 did not accumulate to a high extent in the investigated organs. No considerable signs of toxicity were observed.