The effects of hemofiltration (HF) on serum levels of intact parathyroid hormone (PTH), as well as its mid-C regional and C-terminal fragments and bone Gla-protein (bone gamma-carboxyglutamic acid, osteocalcin), were investigated in 17 patients during one session of HF. During HF there was a significant decrease in the serum concentrations of both intact PTH (p < 0.01), mid-C regional PTH (p < 0.01) and C-terminal PTH (p < 0.01) as well as in the osteocalcin concentrations (p < 0.01). There was a significant increase in serum calcium (p < 0.05) during the procedure and this increase correlated with the reduction in PTH (r = 0.56, p < 0.05). Mid-C regional PTH and osteocalcin were found in the ultrafiltrate of all patients, while intact PTH was found in the ultrafiltrate of 8 of 17 patients and C-terminal PTH was detected only in the ultrafiltrate of one patient. The data demonstrate that the permeability of HF membranes is high enough to cause filtration of both intact PTH, PTH fragments and osteocalcin. An inhibited secretion of PTH probably played a major part in reducing intact PTH in serum during HF.