BACKGROUND: Delta-like protein 3 (DLL3) is considered to inhibit the Notch pathway in the tumorigenesis of small cell lung cancer (SCLC) and other neuroendocrine carcinomas, making it a potential therapeutic target in the treatment of cancer. Since the soluble form (sDLL3) is expected to be useful for predicting the status of DLL3 expression on tumors, analytical methods to measure sDLL3 are required. RESEARCH DESIGN AND METHODS: Assay methods using ELISA and the SMCxPRO platform were developed to analyze sDLL3 concentration in human serum. The performance of the ELISA was evaluated and the SMCxPRO assay was fully validated, and the comparability of the 2 assays was assessed. RESULTS: The performance of the ELISA was acceptable, and in the SMCxPRO assay validation, all pre-defined validation acceptance criteria were met. The 2 assays were comparable within the range of quantification. Concentrations ranged from below the limit of quantification (<1.00 pg/mL) to 18.0 pg/mL for healthy volunteers and from 1.27 pg/mL to 519 pg/mL for SCLC patients by SMCxPRO assay. CONCLUSIONS: Two sensitive assay methods to measure sDLL3 in human serum were successfully established. These assays have potential as novel blood-based assays to assess the status of DLL3 expression on tumors in humans.