1. Tumour extracts from two patients with humoral hypercalcaemia of malignancy contained material which stimulated adenylate cyclase in chick renal membranes and in rat osteosarcoma cells.
2. Adenylate cyclase-stimulating activity in each system was inhibited by a specific parathyroid hormone (PTH) antagonist.
3. Studies in two HPLC systems suggested that the adenylate cyclase-stimulating factors extracted from these tumours differed from each other and from synthetic human parathyroid hormone 1–34.
4. The presence of similar PTH-like adenylate cyclase stimulating material(s) in oncogenic osteomalacia suggests that adenylate cyclase stimulating factor(s) may not be the direct or the sole cause of hypercalcaemia.