Inhibins are gonadal glycoprotein hormones belonging to the transforming growth factor-beta superfamily. They are dimers composed of a common α-subunit and either a βA or a βB subunit, forming either bioactive inhibin A or inhibin B respectively. They play a major role in the hypothalamus-pituitary-gonad axis regulating spermatogenesis and folliculogenesis through negative feedback on FSH secretion. Total inhibin (inclusive of all bioactive forms and free α subunits) is an excellent marker for granulosa cell tumours (GCTs) which are the most common type of ovarian sex cord stromal cancer. This is especially true for postmenopausal women with GCT as inhibin levels are generally undetectable in healthy postmenopausal women. GCT have a tendency for late recurrence, often many years after initial diagnosis; inhibin is valuable for monitoring for ovarian cancer recurrence. An efficient and cost-effective diagnostic assay for total inhibin is not available, underscoring a need to develop a high-throughput total inhibin assay. Mass spectrometry in clinical endocrinology is playing an increasing role in improving the clinical management of numerous endocrine diseases. Our aim is to establish a methodology using selective reaction monitoring (SRM)-based targeted proteomics using liquid chromatography-mass spectrometry (LC-MS) to detect total inhibin with enhanced sensitivity in serum. Using conditioned media from HEK293 cells that overexpress inhibin B, we successfully detected inhibin α peptides using a timsTOF Pro mass spectrometer. STPLMSWPWSPSALR was the highest-scoring detectable peptide sequence at 74.26 (2142.1156 second retention time). Using these parameters, in conjunction with an immunoaffinity enrichment method, we are currently testing serum samples collected from women who have had GCT and comparing them to a standard inhibin α ELISA assay. This study will provide women with GCT access to an inhibin assay with high sensitivity and specificity with the aim to achieve early detection or monitoring of recurrence and an improved overall survival rate.