An apparatus and method are provided to rapidly thaw and heat frozen bags of biological materials, such as plasma. The apparatus has a hollow bladder having a flexible wall that is placed in contact with the bag. A circulation system draws fluid from a reservoir and fills the bladder with the fluid. The circulation system also drains fluid from the bladder. A heater in the reservoir heats the fluid before the fluid enters the bladder. As heated fluid flows through the bladder, heat is transferred through the bladder wall to the plasma bag to thaw the biological material. The bladder wall expands against the plasma bag as the bladder fills with fluid, and contracts from the plasma bag as the bladder is drained. The expansion and contraction of the bladder wall agitates the plasma bag and biological material to accelerate the thawing process.