Platelet concentrate (PC) transfusions are useful for maintaining haemostasis in a variety of clinical situations. The function of transfused platelets is of critical importance, and changes on storage of buffy coat-prepared PC may influence their haemostatic potential. Total platelet adenine nucleotide content and platelet aggregation responses were studied, serially, in pooled buffy coat-derived PCs (n = 7), stored under UK recommended blood bank conditions, over the stipulated shelf-life of 5 days. Mean platelet volume (MPV), platelet counts and platelet distribution width (PDW) were also quantified. Total platelet ADP content decreased from 4.45+/-0.78 to 3.71+/-0.69 nmol/108 platelets (P<0.01, day 1 versus day 5, mean +/- SEM) over the shelf-life period. This was associated with reduced aggregatory responses: responses (expressed as percentage of maximum height) to 5 and 10 microM ADP decreased from 10.8+/-2.8% to 1.0+/-1.0% (P<0.005, 5 microM, day 1 versus day 5) and from 18.0+/-5.4% to 4.7+/-2.2% (P<0.02, 10 microM, day 1 versus day 5) while the decreased responsiveness was more pronounced for 4 microg/ml of collagen: 49.0+/-13.3% to 7.2+/-7.1% (P<0.01, day 1 versus day 4) and 49.9 +/-13.3% to 2.1+/-1.9% (P<0.001, day 1 versus day 5). These data indicate an acquired storage pool defect that is maximal by day 4 or 5 and accompanied by decreased platelet function, characterized by significant decreases in platelet aggregation responses. Addition of freeze-thawed plasma (autologous day 1) to PCs on days 2, 3, 4 and 5 did not alter the responses to ADP and collagen.