The structure-activity relationship of 3-(p-substituted benzoyl)pyrazolo[1, 5-a]pyridine derivatives as inhibitors of rabbit platelet aggregation in vitro have been examined. The cluster analysis approach to hte selection of substituents on the phenyl ring of 2-isopropyl-3-(p-substituted benzoyl)pyrazolo[1, 5-a]pyridine was used, and the effect of substituents was investigated by quantitative regression analysis. The p-dimethylamino group was found to be the most effective p-substituent on the phenyl ring for antiaggregant activity.