Human blood platelets, the richest known source of β-Transforming Growth Factor extractable under acid conditions, release in neutral extracts (pH 7.2) a latent form of this growth factor with an apparent molecular weight of 400 K. This latent form, poorly active on rat NRK-49F indicator cells in soft agar assays can be activated by exposure to acid pH or & molar urea. The acid activated β-Transforming Growth Factor from neutral extracts elutes on Biogel P60, in 1 molar acetic acid, as a broad peak of apparent molecular weight 15–30 Kd, like when this factor is extracted from platelets by the usual acid-ethanol procedure. Moreover, β-Transforming Growth Factor from both acid activated neutral extracts and from acid-ethanol extracts elutes on reverse phase at 30% acetonitrile. We suggest that β-Transforming Growth Factor is stored in human blood platelets as a poorly active high molecular weight complex which may be dissociated and activated in appropriate in vivo microenvironments.