[en] The purpose of this study was to determine a practical method for labelling canine and human blood platelets with In-111-oxine. Standard accepted techniques were applied. The influence of various factors such as pH, labelling medium, incubation time, platelet concentration and oxine concentration on labelling efficiency and in vitro and in vivo platelet viability and ultrastructure was assessed. Physiological saline is an adequate medium for labelling human as well as canine plasma with high efficiency. Labelling was optimal at pH 6.3. - 7.0. Oxine is not toxic to platelets even at concentrations up to 400 mg/l. Since In-111-oxine is dissolved in solutions containing ethanol, and the latter is a known platelet toxin, the maximum permissible oxine concentration was found to be 1.04 mg oxine/l platelet rich saline. This is equivalent to a 0.1 per cent ethanol concentration. The optimal oxine to platelet concentration was found to be 3.06 plus minus 0.8μg oxine/10⁹ platelets. An incubation time of 30 min. was satisfactory. Platelets labelled with In-111-oxine were aggregated with Ristocetin and the amount of radionuclide released from the platelets measured. This was found to be neglible (less than 1 per cent). The viability of labelled platelets was assessed. In vitro platelet aggregation with ADP and collagen was normal. Labelled platelets were axamined with transmission electron microscopy. Although procedures induced ultrastructural features of platelet activation, these morphological changes were largely reversible. These results made it possible to establish an optimum, simple and reliable method for labelling canine and human platelets with In-111-oxine. The platelets are viable and results of in vivo and in vitro studies establish that In-111-oxine is a satisfactory radiolabel. The superior physical characteristics of In-111 if compared to those of Cr-51 the standard radiolabel for platelets, suggests that the former will become the radionuclide of choice for platelet kinetic studies