AbstractTransforming growth factors β belong to a group of cytokines that control cellular proliferation and differentiation. Five isoforms are known that share approximately 75% sequence identity, but exert different biological activities. The structure of TGF‐β3 was solved by X‐ray crystallography and refined to a final R‐factor of 17.5% at 2.0 Å resolution. Comparison with the structure of TGF‐β2 (Schlunegger MP, Grütter MG, 1992, Nature 358:430‐434; Daopin S, Piez KA, Ogawa Y, Davies DR, 1992, Science 257:369‐373) reveals a virtually identical central core. Differences exist in the conformations of the N‐terminal α‐helix and in the β‐sheet loops. In TGF‐β3, the N‐terminal α‐helix has moved ≈︁ 1 Å away from the central core. This movement can be correlated with the mutation of Leu 17 to Val and Ala 47 to Pro in TGF‐β3. The β‐sheet loops rotate as a rigid body 9° around an axis that runs approximately parallel to the dimer axis. If these differences are recognized by the TGF‐β receptors, they might account for the individual cellular responses. A molecule of the precipitating agent dioxane is bound in a crystal contact, forming a hydrogen bond with Trp 32. This dioxane may occupy a carbohydrate‐binding site, because dioxane possesses some structural similarity with a carbohydrate. The dioxane is in contact with two tryptophans, which are often involved in carbohydrate recognition.