We have shown previously that a bispecific antibody (BsAb) directed against both germ-cell alkaline phosphatase (GCAP) and the CD3 complex on mouse T cells could effectively eliminate GCAP-positive tumor cells in vivo using an immunocompetent mouse model. However, some GCAP-negative tumor cells were still able to grow, suggesting that BsAb therapy, when used in a clinical setting, could benefit from targeting several tumor markers to prevent outgrowth of tumor cells lacking a targeted marker. To test this hypothesis, we developed an in vitro model based on primary human ovarian carcinoma (OC) cultures and BsAbs directed against human T cells and several tumor markers [placental alkaline phosphatase (PLAP), GCAP, folate-binding protein (FBP) and CA19.9]. OC cells, isolated from primary tumors, were co-cultured with human peripheral blood mononuclear cells in the presence or absence of various concentrations of BsAbs against PLAP/GCAP, FBP and CA19.9 administered separately or in combination. Results derived from 18 primary OC samples showed that the combination treatment was better than or equally effective as the best single BsAB treatment in 60% of cases. Sometimes targeting FBP, PLAP/GCAP or CA19.9 alone was superior to targeting all simultaneously. Combining each BsAb with a low dose of IL-2 was always beneficial. These results indicate that before using a specific BsAb in the clinic, it is important to determine the optimal BsAb for each patient using this in vitro assay on cells from the removed tumor mass.