Patients with uremia have excess levels of parathyroid hormone (PTH). It has been reported that excess PTH is associated with bone marrow fibrosis. The present study was undertaken to examine the in vitro effect of intact 1–34 PTH and the active 1–34 N-terminal fragment on human bone-marrow-derived fibroblasts. Proliferation of fibroblasts was not stimulated by PTH in concentrations present in uremia (5–30 U/ml) nor by uremic sera which contained either high or low PTH levels. The lack of stimulation contrasted with the activity of both PTH preparations in other systems, i.e., inhibitory to erythroid colonies and stimulatory to heart muscle cells.