The complement (C)-activating capabilities in human serum of 32 mouse and 10 mouse/human chimeric MoAbs of different isotypes, and their fragments, were tested in vitro. Activation of C via the classical pathway (CP) was performed in 1% factor D-deficient serum in gelatin containing Veronal buffer in the presence of calcium and magnesium (GVB++), while activation of the alternative pathway of C (AP) was assessed in 10% C1q-depleted serum in the presence of 5 mM MgCl2 in GVB++. The C-activating ability of MoAbs was expressed relative to the degree of activation of complement by aggregated IgG for the CP and relative to mouse IgG1 for the AP. All of seven mouse IgG2a MoAbs were potent activators of the CP. The results of CP activation by IgG1, IgG2b and IgG3 isotypes were different for individual MoAbs. Only three (two IgG1 and one IgG3) of 32 mouse MoAbs were potent activators of the AP. IgG2a and IgG2b were relatively poor AP activators. There were a few MoAbs which activated both the AP and CP. Of 10 chimeric MoAbs, two IgG1, one IgG2 and one IgG4 were poor or non-activators of the CP. On the other hand, IgG2 and IgG4 were good AP activators. IgG3 was the most potent AP activator. Most of the F(ab')2 fragments were activators of the AP and displayed no activation of the CP. Fc fragments only activated the CP, whereas Fab' did not activate the CP or the AP. These studies suggest that the route of complement activation by class and subclass MoAbs can not always be predicted in advance and based only on their subclass identity.