Hollow Fiber bioreactors are the method of choice for the production of conditioned medium. The molecular weight cut off of the fiber gives you a first, rough idea of the size of the factor of interest. If it shows up in the reservoir bottle it is smaller than 20kd, if it is concentrated in the ECS then it is larger than 20kd. The use of CDM-HD eliminates contaminating proteins from serum making factor identification simplified. Large numbers of cells can be cultured at one time generating large quantities of concentrated conditioned medium supernatants.