BACKGROUND Buffy-coat processing allows for the use of platelet additive solutions (PASs). PASs reduce plasma-associated transfusion reactions and conserve plasma for transfusion or fractionation. Platelet (PLT) storage in plasma was compared to storage in three commercially available PASs compared to assess their influence on in vitro laboratory variables. METHODS Platelet concentrates (PCs) were prepared from leukoreduced pools of four buffy coats (BCPs) suspended in autologous plasma or one of PASs (Composol, Fresenius-Kabi; T-Sol, Baxter Corp.; or SSP+, MacoPharma). On Days 1, 2, 3, 5, and 7 of storage, samples were tested for PLT concentration, mean PLT volume (MPV), CD62P, morphology, pO2, pCO2, glucose, lactate and total protein concentration, pH, extent of shape change (ESC), and hypotonic shock response (HSR). Data were analyzed by analysis of variance (ANOVA) with repeated measures and t tests. RESULTS PLT recoveries from BCPs were higher (p < 0.05) with plasma than any PAS. Storage medium and duration did not affect PLT concentration or MPV over time. CD62P expression and morphology were significantly different among PCs pooled with different media. ANOVA showed (p < 0.05) differences among the rates of change of pCO2, pH, glucose consumption, lactate production, and ESC; PASs such as Composol and SSP+ offered excellent maintenance of pH and low rates of glucose consumption. PAS performed poorly in ESC and HSR compared to plasma. Correlation studies reveal far more significant correlations between variables of PLTs in PAS than in plasma. CONCLUSIONS Newer PASs, for example, SSP+ and Composol, can maintain PLT integrity and moderate metabolism similarly to plasma but offer consistently lower PLT recoveries and limited osmotic balance.