Summary.Blood was collected from nine normal volunteers into each of four different anticoagulants and in order to simulate transport conditions, was stored at room temperature for 96 h. The platelet associated IgG (PAIgG) was determined from aliquots thereof over this period. Blood in EDTA gave slightly higher initial values (day O) than in other anticoagulants. PAIgG levels increased at different rates in all anticoagulants thereafter. In contrast to recent reports which suggested that falsely elevated levels were likely to be seen after storage in EDTA, we found little difference in these values in blood in different anticoagulants at 72 h and all PAIgG measurements remained within our quoted normal range at this time. After 96 h storage however, one of nine (11%) in CPD‐A, two of nine (22%) in EDTA, two of nine (22%) in Na‐citrate and six of nine (66%) in EDTA paraformaldehyde gave falsely elevated results. The possible mechanisms of these changes are discussed. The assay system employed in this study measures ‘total’ platelet IgG in a platelet extract in contrast to some other assays which quantify surface platelet IgG alone and it is possible that this difference in technique is responsible for the relatively smaller percentage changes after storage than reported by others. All anticoagulants tested in this study proved satisfactory both for handling and for the measurement of PAIgG at 48 h, with the proviso that normal ranges should be established f