SUMMARY.A technique for the labelling of human blood platelets with radioactive sodium chromate is described, and is compared with those previously in use. The uptake of radiochroinate by platelets is greatest when the platelet suspension is of small volume, plasma protein is absent, and the sodium chromate concentration is 0-2 to 3-0 ng. per lO'" platelets. The rate of the reaction is not a£fccted by its duration up to 40 minutes, but is enhanced by an increase in temperature. About one-third of platelets in suspensions labelled as described are viable, and survive in the circulation for 8-10 days.

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