The subject invention provides a plant cell culture for producing proteinaceous agents comprising a plant cell line stably transformed to express a transgene encoding a proteinaceous agent and a growth medium which supports the growth of said plant cell culture but which does not support the growth of Mycoplasmataceae and contains no materials of animal origin. The plant cell line is capable of being continuously passaged such that consistent transgene expression is maintained during passaging. The plant cell line is also capable of being cryopreserved such that consistent transgene expression is recovered upon recovery from cryopreservation.