Recent studies suggested that the expression of P-Selectin on stored platelets is related to in vitro activation and loss of viability. We examined the effects of dimethylsulfoxide (DMSO) on in vitro function and P-Selectin expression of platelet concentrates. Fresh random-donor platelet units (n = 60) were divided into four equal groups. A DMSO-free group was chosen as a control. DMSO (0.5%, 1.0%, and 3.0%) was added to the other three groups. The samples were stored on a horizontal shaker at room temperature. Biochemical, morphological and platelet function tests and P-Selectin expression were monitored during storage. In all groups, P-Selectin expression, lactate and LDH levels, mean platelet volumes and PO2 increased but the aggregation response to agonist, the recovery response to hypotonic shock, platelet count, glucose level, pCO2, and HCO3 decreased during storage. In DMSO-containing groups, the P-Selectin expression which is a predictor of in vitro activation, was found significantly less often than in the DMSO-free group.