Vented flasks incorporate a 0.2 µm microporous hydrophobic membrane filter in the plug-seal screw cap to prevent cross-contamination and ensure consistent gas exchange. The vented screw cap eliminates media leakage and gas blockage problems associated with “loosened cap” procedures. Flask volume is proportional to the vent surface to maintain equivalent rates of gas exchange. Flasks are made from optically clear polystyrene, treated to promote optimum cell growth and attachment. Sterilized by gamma irradiation.