KRDS (Lys-Arg-Asp-Ser), a tetrapeptide from human lactotransferrin, was tested in vitro on human platelet function, and its effects were compared to those of RGDS, a tetrapeptide from human fibrinogen. Both peptides had a high probability of initiating a β-turn and were highly hydrophilic. KRDS inhibited ADP-induced platelet aggregation [median inhibitory concentration (IC50) 350 μM] and fibrinogen binding (IC50 360 μM) to a lesser extent than RGDS (IC50 75 μM and 20 μM, respectively). Different from RGDS, thrombin-induced serotonin release was inhibited by KRDS (750 μM) on normal platelets (55 ± 10%) and type I Glanzmann's thrombasthenia platelets (43%± 1). However, KRDS had no effect on cytoplasmic Ca2+ mobilization, inositol phospholipid metabolism or protein phosphorylation (myosin light chain P20 and P43). In contrast to RGDS, KRDS does not inhibit the binding of monoclonal antibody PAC-1 to activated platelets. KRDS and RGDS inhibited 4β-phorbol-12-myristate-13-acetate (PMA)-induced aggregation and fibrinogen binding, while proteins were normally phosphorylated. Thus, the tetrapeptide KRDS is (a) an inhibitor of serotonin release by a mechanism independent of protein phosphorylation and (b) an inhibitor of fibrinogen binding and, hence, aggregation by a mechanism that may not necessarily involve its direct binding to the glycoprotein IIb-IIIa-complex.