We examined whether or not parathyroid hormone-related protein (PTH-rP) is produced in cultured human myeloma cells. PTH-rP protein was detected in the cell extract of one (U266) of 6 myeloma cell lines examined by radioimmunoassay. Also we demonstrated the expression of PTH-rP mRNA in all of the 6 lines. These findings indicate that PTH-rP may be one of the factors responsible for the bone destruction or hypercalcemia in multiple myeloma.