Mouse thymocytes are known to undergo apoptosis by ligating some unique anti. Thy-1 monoclonal antibodies (mAbs), G7 and KT16. However, the precise mechanisms of Thy-1-mediated apoptosis are as yet unclear. We investigated Thy-1-mediated apoptosis using our previously generated anti-Thy-1 mAb, MCS- 34, which was similar to G7 because both antibodies recognized both Thy-1.1 and Thy-1.2 and bound Thy-1A epitope. Unlike G7, MCS-34 alone could not induce apoptosis in thymocytes; however, it could induce apoptosis when it was cross-linked with second antibodies. Thus, MCS-34 could not aggregate by itself, but G7 could. In the course of investigating the apoptosis-related molecules that were involved in the thymocyte apoptosis induced by cross- linking of MCS-34 or by G7 ligation, we found that CPP32-like proteases were activated during the apoptosis. Furthermore, the expression of bcl-2 and bcl-X(L) proteins was decreased in these apoptosis processes. Whereas the ligation of MCS-34 alone could not generate apoptosis signals that led to the activation of CPP32-like proteases and the decrease in bcl-2 and bcl- X(L) expression, the aggregation of Thy-1 glycoprotein might be crucial to signal thymocyte apoptosis. These results indicate that MCS-34 is a useful anti-Thy-1 mAb for analyzing the Thy-1-mediated signals since MCS-34 can control the level of apoptosis by using second antibodies.