SYNOPSIS Platelet survival was determined using untreated and siliconed glass bottles and plastic bags (Fenwal) for collecting and storing blood. The platelets were tagged in vivo with p32 in six polycythaemic patients undergoing treatment with P32. The results showed that fresh ACD blood collected in untreated glass, siliconed glass, and plastic gave the same recovery of platelets in the recipients. The use of EDTA (Fenwal formula) as anticoagulant gave results inferior to those obtained with blood using ACD as anticoagulant. Even after storage up to 24 hours in untreated glass bottles (ordinary bank blood) a satisfactory recovery of platelets was observed. After storage for 72 hours the recovery was less but not negligible.We have tried to solve part of the problem of preserving platelets for subsequent transfusion and for use in extracorporeal circulation by determining the survival of platelets in blood delivered in different containers, using ACD and EDTA as anticoagulant and storing the blood for periods of varying length before transfusion. It is difficult to determine the survival of thrombocytes but it is possible by using platelets tagged with various isotopes. A survey of the methods used was recently published by Aas and Gardner (1958). Tagging is usually performed in vitro, and the rather rough treatment to which the platelets are exposed during tagging has some influence on the results obtained, consequently labelling in vitro is not ideal for an investigation in which a relatively small difference in survival time might indicate superiority or inferiority of one container compared with another. We therefore used platelets tagged in vivo with p32. We used blood from patients with polycythaemia who were treated with p32. The technique employed is very similar to that used by Adelson, Rheingold, and Crosby (1957) in dogs and human beings. It is well known that tagging is maximal in about a week after the administration of the drug (Adelson et al., 1957). Using blood from patients with polycythaemia gives another advantage, namely, the possibility of drawing several pints of blood from the same donor at very short intervals. The various 'pints' drawn from the same donor contain the 'same' platelets marked in exactly the same way in vivo, thus affording an ideal opportunity for comparing the effect of storage on platelets in various containers before transfusion. MATERIALS AND METHODSDONORS Patients with polycythaemia undergoing treatment with P32 were used as donors. They were given about 5 'a. P32 by mouth and after an interval of five to seven days bleedings were started, 500 ml. of blood being drawn on each occasion, and the bleedings were continued daily for three to six days, depending on the clinical and haematological condition of the patients. Six