SARS-CoV-2 spike proteins induce complement-mediated cell killing that can be blocked by complement inhibitors in the mHam assay. TF1PIGAnull cells were treated with 20% NHS preincubated with diluted SARS-CoV-2 spike protein subunit 1 (S1), subunit 2 (S2), N proteins (N), and HCoV-OC43 S proteins (2.5 µg/mL to 20 µg/mL), and then measured for cell killing. Complement-mediated cell killing (%) was markedly increased in a dose-dependent manner with addition of S1 (A) and S2 (B). Increasing the concentration of N protein (C) and HCoV-OC43 S protein (D) did not increase the cell death (%) from baseline NHS level. Complement inhibition with 1 µM factor D inhibitor or 50 µg of anti-C5 monoclonal antibody completely blocked the cell killing induced by 20 µg/mL S1 (E) and S2 (F). The dotted line at 20% nonviable cells was established as a threshold for a positive mHam based on a receiver operative curve. All experiments were repeated at least 3 times. Statistical significance was calculated between each CoV protein–treated group and the NHS-treated group (*P < .05, **P < .01, ***P < .001, ****P < .0001). Anti-C5Ab indicates anti-C5 monoclonal antibody; FD Inh, factor D inhibitor (ACH145951); NHS(H), heat-inactivated NHS; NS, not significant.