We analyzed human T cell lymphotrophic virus type I (HTLV-I)-infected T cells for the presence of mRNA coding for parathyroid hormone-related protein (PTHrP) by Northern blotting using synthetic DNA probes. We report here that PTHrP mRNAs were detected in a HTLV-I-infected T cell line, MT-2, but not in uninfected T cell or B cell lines, and that PTH-like bioactivity was detected only in the conditioned medium of MT-2 cells. Our study suggests that the pathophysiology of hypercalcemia in patients with adult T cell leukemia/lymphoma may resemble that which occurs with solid tumors.