The effects of tumor-conditioned media (TCM) derived from cultured cells from an oncogenic hypophosphatemic osteomalacia (OHO) tumor on transformed human kidney cells were investigated. Dose-dependent cell detachment and aggregation occurred in kidney cells cultured in serum-free medium supplemented with TCM, but not in skin fibroblast controls, or in kidney cells cultured in the presence of serum. Kidney cells exposed to TCM in the presence of serum (0.5%) had reduced Na(+)-dependent phosphate cotransport (36%, p < 0.04) and increased 1alpha-hydroxylase activity (48%, p < 0.05). In contrast, TCM had no significant effect on Na(+)-dependent alpha-methyl-glucose transport. To investigate these effects further, serum from an OHO patient, before and after tumor resection, was used to raise polyclonal antiserum to tumor-derived products (preoperative and postoperative antiserum, respectively). Changes in Na(+)-dependent phosphate cotransport and vitamin D metabolism induced by TCM were prevented by the addition of preoperative but not postoperative antisera. Furthermore, Western analysis revealed the presence of two proteins (56-58 kDa) in TCM media screened with preoperative antisera. These proteins were not detected by postoperative antisera and were absent in skin fibroblast control media. Direct inhibition of Na(+)-dependent phosphate cotransport by phosphonoformic acid did not affect 1,25-dihydroxy vitamin D(3) synthesis. These studies provide support for a circulating component affecting phosphate handling and vitamin D metabolism in OHO.