In order to detect the anti-tumoral activity of HER2/CD3 BsAb, the HER2-positive breast cancer tissue samples from six patients were treated with RPMI 1640 medium alone or RPMI 1640 medium containing 0.1 μg/ml or 1 μg/ml HER2/CD3 BsAb and the tumor growth was determined using volume and weight measurements. This evidence concerns the gene ERBB2 and breast carcinoma.