The huHMFG-1 (AS1402) antibody is a humanised IgG1 directed against MUC1 and is currently in clinical trials for the treatment of breast carcinoma. Adenocarcinomas over-express and shed MUC1, and high MUC1 serum levels are associated with progressive disease. Here, we have investigated the effects of MUC1 present in sera from breast and ovarian cancer patients and that of NK cells on in vitro huHMFG-1-mediated ADCC, performed with and without the addition of various cytokines. Screening for patients with high levels of NK cells bearing the FcgammaRIIIa-158V polymorphism, adjusting the dosage to circulating levels of MUC1 and co-administration of NK cell activating cytokines may increase the efficacy of huHMFG-1 treatment.