Antibody-drug conjugates (ADCs), which combine the exquisite specificity of antibodies with the cell-killing ability of cytotoxic drug payloads, have emerged as an attractive means for treating cancers. All (pre)clinical ADCs employ biosynthesized cytotoxins as their ADC payload precursors (APPs). The cost-effective bioproduction of APPs is receiving great interest from both academia and industry. Given the lack of systematic overviews of the topic, we provide the current status of APPs and focus on their state-of-the-art bioproduction strategies, illustrated with typical examples and critical analyses. Challenges in further enhancing the bioproduction efficiency of APPs and other cytotoxins are also discussed. This research has implications for bioprocess and metabolic engineering, systems and synthetic biology, and biopharmaceutical drug discovery, development, and industrialization.