Abstract: An apparatus is provided for heating contained liquid, and more particula for thawing frozen blood components which are contained in a bag. The heating apparatus includes a microwave oven which has a cavity for receiving objects to be heated. A track is mounted in the oven cavity and has a peak and a valley with respect to the cavity floor. A tray is provided for supporting the liquid container. A device is provided for pivotally supporting the tray within the cavity with the tray in engagement with the track means, and a device is located within the oven for imparting relative motion between the tray and the track. With this arrangement the liquid container can be moved in a rocking motion to cause a mixing between the warm and cold portions of the liquid, and in particular with regard to frozen blood will cause the thawed liquid portions of the blood to flow over the still frozen portions so as to dissipate heat therebetween.