The glycoprotein prosaposin has previously been demonstrated to inhibit tumour metastasis in lung and breast cancer models through stimulation of monocyte production of the antitumorigenic protein thrombospondin 1 (TSP1). Here, Wang et al. report that a cyclic peptide derived from the active sequence in prosaposin promotes tumour regression in a patient-derived tumour xenograft model of metastatic ovarian cancer. Analysis of tumour tissue from 134 patients with serous ovarian cancer revealed that 97% of the tumours expressed CD36, the receptor that mediates the pro-apoptotic activity of TSP1.