Abstract: A holder assembly for a plastic bag of a frozen liquid, e.g. blood plasma, is designed to be manually removably mounted on an arbor that projects into a microwave oven cavity from a wall thereof. The holder is metallic and has a circular base from which a peripheral wall projects upwardly to a rim across which an apron extends. A semi-flexible bar of microwave transparent material is mounted to extend across the holder to retain the bag therein. A plurality of such arbors, e.g. four, are disposed around the oven cavity whereby a number of bags can be heated simultaneously. Each arbor is rotatable for agitating the bag holder assembly and hence the bag therein. Devices associated with each arbor detect the presence of a bag holder assembly on such arbor and also detect the temperature of thawing liquid in the bag supported in each bag holder assembly. A controller to which this information is supplied drives the oven through an appropriate program that ensures that none of the bag contents overheats.