An assessment was made of the serum protein binding of representative oral cephems (cefdinir, cefixime and ceftibuten) for sera from healthy subjects (HS) and patients with chronic renal failure (CRF) using an application of equilibrium dialysis under a same set of conditions in vitro. The protein binding capacity of oral cephems in CRF patients being treated with continuous ambulatory peritoneal dialysis (CAPD) or hemodialysis (HD) was significantly less than that in HS, and a marked increase in free drug concentration was observed. While examining the protein binding of oral cephems with heparin in patients on HD, binding capacity decreased significantly immediately following the completion of dialysis compared to that prior to dialysis. On the other hand, the protein binding of oral cephems did not change when used nafamostat mesilate as an anticoagulant. The addition of palmitic acid (PA), a common non-esterified fatty acid (NEFA), to pooled sera from HS caused the binding capacity of oral cephems to decrease, accompanied by increase in PA concentration. It appears from these findings that changes in the binding capacity of oral cephems with HD have possibly been caused by increase in NEFA due to activation of lipase when heparin was used as an anticoagulant. In conclusion, changes in the protein binding capacity of oral cephems in CRF patients should be taken into consideration in attempts to avoid possible side effects.