Monoclonal antibodies (mAbs) that exert antitumor activity can do so by virtue of their effector function and͞or their ability to signal growth arrest or cell death. In this study, we demonstrate that mAbs which have little or no signaling activity-i.e., anti-CD19, CD20, CD21, CD22 and Her-2-can become potent antitumor agents when they are converted into IgG-IgG homodimers. The homodimers exert antigrowth activity by signaling G 0 ͞G 1 arrest or apoptosis, depending upon which cell surface molecule they bind. This activity is specific and, in the case of the anti-CD19 mAb, did not require an Fc portion. These results offer the possibility that homodimers of other tumor-reactive mAbs which have little antitumor activity as monomers might be potent, antitumor agents.