In the present study, the efficacy of a new drug, i.e. the bispecific single-chain antibody MT110 targeting the epithelial antigen EpCAM and the T-cell antigen CD3 was tested ex vivo in malignant pleural effusions (MPEs). EpCAM+ epithelial cells were found in 78% of the MPEs (n = 18). Ex vivo treatment of seven MPEs resulted in a dose-dependent specific lysis of 37 +/- 27% (+/- SD) EpCAM+ cells with 10 ng/ml (P = 0.03) and 57 +/- 29.5% EpCAM+ cells with 1,000 ng/ml MT110 (P = 0.016) after 72 h. As a prerequisite for redirected lysis, stimulation of the autologous CD4+ and CD8+ cells in MPE by 1,000 ng/ml MT110 resulted in 21 +/- 17% CD4+/CD25+ and 29.4 +/- 22% CD8+/CD25+ cells (P = 0.016, respectively) after 72 h. This was confirmed by a 22-fold release of TNF-alpha and 230-fold release of IFN-gamma (1,000 ng/ml, 48 h, P = 0.03, respectively). Thus, relapsed breast cancer patients resistant to standard treatment might benefit from targeted therapy using MT110.