The action of 3-[4,5-bis-(4-chlorophenyl)-oxazolyl-(2)-thio]-propionic acid (tioxaprofen, EMD 26 644), a non-steroidal antiinflammatory substance, on platelet function has been studied in vitro and ex vitro. Collagen-induced aggregation and the second phase of adrenaline-induced aggregation are inhibited by small concentrations of tioxaprofen, The ADP-induced aggregation being inhibited only by higher concentrations. The generation of malonedialdehyde (MDA) is blocked. The pathological spontaneous aggregation found by the method of Born was reversed after treatment, and the pathologically increased aggregation according to platelet aggregation test (PAT I) of Breddin was normalized. Tioxaprofen blocks the formation of thromboxane most probably by inhibition of cyclo-oxygenase, but an inhibition of thromboxane synthetase cannot be excluded by our results. The in vivo action starts immediately after ingestion of tioxaprofen and is maintained by small doses. By checking the routine parameters of the blood a slight increase of creatinine and blood urea nitrogen (BUN) and a slight reduction of gamma-GT and alkaline phosphatase were found, all values remaining within the reference interval.