Binding to HSA has been determined for diflunisal (alpha = 0.03%), diclofenac (0.09), ibuprofen (0.10), bumadizone (0.11), ketoprofen (0.14), oxyphenbutazone (0.28), indomethacin (0.39), mofebutazone (0.57), tenoxicam (0.59), piroxicam (0.91), salicylic acid (1.00), o-carbamoylphenoxyacetic acid (11.58) and salicylamide (24.91). The free concentration of phenprocoumon was raised by diflunisal up to 35% in a dose dependent manner. Ibuprofen and piroxicam did not show significant effects. It is concluded that diflunisal is bound only slightly to the phenoprocoumon binding site of HSA while ibuprofen has no affinity to this part of the albumin molecule.