The study of novel platelet aggregation inhibitors is important for the development of more effective and selective antithrombotic agents. In this study, we compared the inhibitory activity of compounds—derivatives of 3-phenyl-2-isoxazoline-5-carboxamide containing three to five fluorine atoms in the aromatic substituent (10 new compounds)—and the reference drug clopidogrel against adenosine diphosphate (ADP)- and collagen-induced platelet aggregation. For each compound, the percentage of platelet aggregation inhibition was measured over a concentration range of 1–25 mM upon activation with ADP and collagen using light transmission aggregometry. All newly obtained compounds demonstrated a dose-dependent effect but were less effective when compared with clopidogrel. Compound 5 exhibited consistently high non-selective activity (62% and 63.5%). Compounds with selective action were identified: compounds 3, 8, and 10 were more effective at inhibiting ADP-dependent aggregation (difference of 20%–30%), whereas compounds 4, 6, 7, and 9 were more effective against collagen-induced aggregation (difference of up to 28%). The obtained data indicate the existence of compounds with a selective action profile against different platelet activation pathways despite having nearly identical structures. The most promising compounds for further research are compound 5 (broad spectrum), compound 7 (collagen-selective), and compound 10 (ADP-selective).