Platelets were labeled with a new neutral, lipid-soluble metal complex of 111In and tropolone. Unlike oxine, which is soluble in ethyl alcohol, tropolone is soluble in isotonic saline. Platelet labeling with 111In-tropolone can be performed in both acid-citrate-dextrose (ACD) plasma and ACD saline within 2 h. Labeling efficiency was 80-90%. 111In-tropolone in ACD saline and ACD plasma at tropolone concentrations of 5 and 10 .mu.g/ml, respectively, and incubation of the platelets with the tracer at room temperature for 20 min were optimal conditions for labeling. An ACD-saline kit was developed for convenient preparation of 111In-labeled platelets. No adverse effect of 111In-tropolone on platelets was observed in studies of biodistribution, recovery and survival of platelets in rabbits and dogs.