Chinese hamster ovary (CHO) cells are widely used for the production of recombinant proteins for clinical use as well as academic research. They are particularly important for the production of glycoproteins where bacteria cannot be used. TGF beta 1 is a potent cytokine highly conserved across species with multiple immunological and non-immunological effects. We have discovered that CHOK1, the CHO clone most commonly used by the pharmaceutical industry, constitutively secretes latent TGF beta 1 and that this hamster TGF beta 1 is active on human cells inducing profound immunological effects. As far as we are aware, the production of TGF beta 1 by CHOK1 cells has not been reported before in the literature. As TGF beta 1 exerts powerful and pleiotropic effects on diverse cell types, and as CHO cells are used to produce a large number of clinical and non-clinical products, our findings are highly relevant to studies that rely on recombinant proteins. Biotechnol. Bioeng. 2011; 108: 2759-2764. (C) 2011 Wiley Periodicals, Inc.