Mature dogs were vaccinated with a myxovirus vaccine. At this time, half of the dogs were also given a subcutaneous injection of levamisole hydrochloride (2.2 mg/kg). Blood platelets were counted daily. The dogs that had received only the vaccine showed, after 48 hours, a 48% decrease in platelets. Levamisole reduced the thrombocytopenia associated with myxovirus vaccination. Levamisole could have a protective effect on platelets by preventing their aggregation induced by viral neuraminidase.