Platelet concentrates stored at room temperature deteriorate. The so-called storage lesion is characterised by morphological changes and a loss of functionality. To find an assay for early platelet activation in platelet concentrates the morphological score, beta-TG release and P-selectin expression were determined, and compared with the amount of soluble P-selectin. An ELISA was used to quantify soluble P-selectin in the storage medium. We found a significant correlation between the amount of soluble P-selectin and the percentage of P-selectin positive platelets (flow-cytometric analysis) (r = 0.7449; p < 0.0001) or the amount of beta-TG release (r = 0.6837; p < 0.0001). The morphological score also correlated significantly (negative) with the amount of soluble P-selectin (r = -0.7669; p = 0.0002). From day 0 till day 8, the amount of soluble P-selectin increased constantly from 219 +/- 49.2 ng/ml to 556 +/- 102.3 ng/ml. The detection of soluble P-selectin can be used to quantify activation of platelets during storage. The immuno-assay for soluble P-selectin is more sensitive than flow-cytometric analysis of the percentage of P-selectin-positive cells and allows earlier detection of platelet activation.