1. Two methods for the isolation of platelets are described in detail. Employing both methods, 55 isolations were made from 31 human beings and 20 isolations from 20 dogs. Both methods are based on the principle of using the platelet layer obtained by repeated fractional centrifugation.

2. The Blood Anticoagulant Platelet Isolation (A) differs from the Blood Saline Platelet Isolation (B). In Method A the following anticoagulants were used: sodium heparin, in various concentrations; sodium citrate; sodium oxalate; and magnesium sulfate. In Method B 10 to 15 parts of saline were added to 1 part blood to keep the blood incoagulable.

3. Special Platelet Isolation (P-I) tubes were designed to isolate the platelets in the final steps.

4. In both methods the platelets were washed with chilled 0.9 per cent sodium chloride solution. The final platelet suspension in saline was obtained free from red and white cells and plasma after six to seven centrifugations and washings.

5. Method A has the advantage of being less laborious. Method B may be preferred for studies of platelets obtained without anticoagulants.

6. The approximate platelet yield in each method varied greatly. It was found that high concentrations of heparin did not produce the highest yields of platelets.

7. Decreases of more than 25 per cent of the initial count were considered significant. The platelets isolated from blood containing sodium heparin, sodium citrate, and sodium oxalate did not significantly decrease in number following storage for a fairly long time. However, platelets isolated using magnesium sulfate and saline did decrease significantly after one to three days’ storage at 1 to 4°C.

8. The isolated platelets did not appear to be morphologically altered. The cell outline was regular and the size did not vary extremely.

9. Both methods are recommended for securing pure platelet suspensions. Such suspensions should prove to be very satisfactory for studies of the physiology, the chemical constitution, and the physical properties of the mammalian blood platelet.

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