Melatonin exerts immunomodulatory actions that enhance the magnitude and quality of immune responses specific for certain antigens; this has raised the possibility of using melatonin to design novel vaccine adjuvant systems. The present study investigated the effect of subcutaneous slow-release melatonin implants and subcutaneous melatonin injections on the responsiveness of circulating platelets in sheep after vaccination against Dichelobacter nodosus (A1 and C serotypes), the bacterium that causes ovine footrot, a major cause of lameness in sheep. The experiments were carried out in sheep from a farm located in an area of Mediterranean-type ecosystem. Plasma melatonin levels were determined by radioimmunoassay, sheep platelet aggregation was monitored using an aggregometer and Ca2+ mobilization was determined by spectrofluorimetry using fura-2. Administration of melatonin either by implants or subcutaneous injections increased plasma melatonin concentrations, an effect that was found to be greater and more sustained when melatonin was administered via implants. Vaccination per se, as well as melatonin, increased the percentage and rate of platelet aggregation and reduced the lag-time in response to the physiological agonist thrombin, an effect that was found to be significantly greater when melatonin was administered to vaccinated animals. Melatonin enhanced thrombin-evoked Ca2+ release and entry and further increased Ca2+ mobilization observed in platelets from vaccinated sheep. These observations suggest that the use of melatonin, as a novel adjuvant, induces beneficial effects on platelet function and haemostasis, and opens new perspectives for therapeutic manipulation of immune responses to vaccination.