Background signal in pooled cynomolgus monkey serum differs across biotherapeutics in the generic assay. Eight biotherapeutics were added at 5 μg/mL to a pooled cynomolgus monkey serum and tested in the generic assay. The background signal in the presence of biotherapeutic was compared to the signal in the unspiked sample and the percent difference calculated. The background signals differed across biotherapeutics with % differences compared to the unspiked pooled monkey serum ranging between −21% and 31%. ADC = antibody-drug conjugate; mAb = monoclonal antibody.