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. TGFβ1 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 TGFβ1 and that this hamster TGFβ1 is active on human cells inducing profound immunological effects. As far as we are aware, the production of TGFβ1 by CHOK1 cells has not been reported before in the literature. As TGFβ1 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.