New propionic acid derivatives are claimed to induce a lower incidence of gastrointestinal bleeding and hemostatic disturbance than older anti-inflammatory analgesics such as aspirin. One of these (ibuprofen, Motrin) was given (600 mg orally) to normal subjects and hemophiliac subjects on a random, double-blind basis (lactose placebo). Platelet adhesiveness, aggregation, platelet and red cell counts, percent packed cells, percent hemoglobin, serum ibuprofen levels, and modified Ivy bleeding time were measured before and 2 and 24 hr after drug. Predrug and 24-hr postdrug values were normal, but at 2 hr after drug, adenosine diphosphate, epinephrine, and collagen aggregations were inhibited and bleeding times were slightly but significantly prolonged in the ibuprofen-treated normal subjects. The other parameters tested remained unchanged. Similar results were obtained in the ibuprofen-treated hemophiliac patients, although bleeding time was not prolonged. The results suggest that ibuprofen may be given to hemophiliac subjects with greater safety than some of the older anti-inflammatory drugs.