Platelet function studies were conducted on 25 parasitologically positive cases of Indian kala-azar and 25 age and sex matched healthy controls. Ninety-two per cent of patients had thrombocytopenia of variable degree; in 44% of patients, platelets were less than 60,000 mm-3. The platelet adhesive index was less than 30% in 70% of patients with kala-azar (normal 31-60%). Platelet aggregation time with ADP and adrenaline was abnormally prolonged compared to the controls. Platelet factor III availability was poor in 40% of cases. There was a fair degree of correlation between platelet adhesiveness and platelet factor III availability in these patients: 50% of patients with poor platelet adhesiveness showed reduced platelet factor III availability.