Effect of melatonin administration on the percentage of thrombin-induced platelet aggregation in vaccinated and non-vaccinated sheep. Platelets from sheep vaccinated or not against Clostridium perfringens type D and treated with melatonin or the vehicle, as indicated, were stimulated in a medium containing 1 mM Ca2+ with 1 U/mL thrombin. Data are mean ± S.E.M. of 10–12 independent experiments. *P < 0.05 compared with their respective controls.