A 96-well microplate assay that is used to determine the amount of Complex I in a human sample. The enzyme is immunocaptured within the wells of the microplate using a monoclonal antibody, and the amount of enzyme in each well is determined by adding a Complex I-specific monoclonal antibody (against a different epitope) conjugated with horseradish peroxidase. The peroxidase changes the subsequently added development substrate from colorless to blue. This reaction takes place in a time-dependent manner proportional to the amount of protein captured in the wells. The rate of development of blue color can be followed at 600 nm or the reaction can be stopped, at a user defined time, by the addition of 100 µl of 1.5N HCl to each well (not supplied) and read at 450 nm.