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Antibody-drug conjugates (ADCs) consist of cytotoxic agents covalently conjugated to monoclonal antibodies that substantially improve antitumour activity and reduce systemic toxicity. With the growing number of ADCs in clinical applications, more accurate bioanalysis data are urgently needed to facilitate the development and rational use of ADCs. Herein, we used antigen-positive cells as antigen carriers and ofatumumab (OFA-HL) and ofatumumab-based ADC (OFA-HL-MMAE) as examples to establish a new ligand-binding assay (LBA) method based on flow cytometry. We proved that the new method met the required analytical performance criteria and the lower limit of quantitation (LOQ) was 0.2 μg/mL. In addition, the LOQ of the quantitative OFA-HL flow cytometry method was reduced to 0.025 μg/mL by choosing an optimized fluorescent antibody, which indicated that the LOQ of the new method can be improved. What's more, the new method showed good stability and specificity when we used it to determine the concentrations of OFA-HL and OFA-HL-MMAE in mouse serum. During the bioanalysis of ADCs, various factors should be considered. Therefore, choosing optimal methods for ADC bioanalysis is necessary. This new method using in situ antigens not only extends the scope of application of the conventional LBA methods by avoiding the need for soluble antigens, but also improves the authenticity of ADC bioanalysis as a supplementary approach, which is valuable for developing accurate ADC assays.