1. A detailed laboratory method is described for the labelling of human platelets with [ 111 In]indium oxine. The 45 min method is simple, requires only 26 ml of blood and is suitable for routine clinical use. 2. After the labelling and resuspension of the platelets in plasma, aggregation responses to both adenosine diphosphate and collagen were similar to those of normal platelet-rich plasma. Less than 5% of the [ 111 In]indium oxine was released by secretory functions of platelets. 3. Labelling efficiencies of 90·1 ± 4·29% ( n = 28) were achieved in 60 s by normal concentrations of plasma-free platelet suspensions. 4. Platelet survival in vivo in healthy volunteer subjects follows a linear function with a survival time of 8·44 ± 0·18 days.