We have previously reported that platelet concentrates (PC) may be irradiated with ultraviolet light (UVL) in a cryostorage pack such that mixed lymphocyte reactions (MLR) are abolished whilst satisfactory platelet function is retained during subsequent storage using Fenwal PL-1240 containers. We have now studied both platelet structure and function after irradiation in DuPont Stericell bags which are both UV-permeable and biocompatible. The irradiation dosage was 3000 Joules/m2 of UVL at a mean wavelength of 310 nm; a dose previously shown to abolish MLR. No detriment to platelet function was observed when compared to control as measured by aggregation responses to adenosine diphosphate (ADP), collagen and ristocetin, hypotonic shock response and pH during 5 d of storage. Lactate levels were significantly higher (P less than 0.01) and glucose levels lower (P less than 0.01) in UV-treated PC, although in the majority the lactate level did not exceed 20 mmol/l. Betathromboglobulin and platelet factor 4 levels were higher during storage in the UV group, the latter reaching significance (P less than 0.05). When whole platelets and platelet membranes were stained with Coomassie blue or Periodic Acid-Schiff's reagent after electrophoresis in polyacrylamide gels no obvious alterations to major membrane constituents were observed on days 1 and 5 of storage. Paired in vivo autologous studies in healthy volunteers using 111Indium-labelling were performed at the end of 5 d of storage. The UV-treated platelets gave satisfactory and equivalent results for recovery, half life and survival when compared to controls. We conclude that PC irradiated with UVL and stored for 5 d in DuPont Stericell containers appear suitable for transfusion and may prove to be nonimmunogenic. Further in vivo studies of haemostatic efficacy and recipient alloimmunization are now warranted.