Transforming growth factor beta (TGF-β) is a multi-functional regulatory protein which can affect growth, immune responses, angiogenesis and the formation of extracellular matrix. Its role in breast carcinomas has been investigated using an antiserum to TGF-β₁ and immunohistochemistry. 27 ductal carcinomas in situ and 54 invasive carcinomas were examined, employing formalin-fixed, paraffin-embedded material. There was no reactivity in 55.5% of in situ carcinomas in comparison with the invasive tumour where only a third were negative. Prominent reactivity was seen in 11% of in situ tumours, and 20% of invasive carcinomas. There was no correlation between detection of transforming growth factor β₁, and histological grade, oestrogen receptor status, epidermal growth factor receptor status and Ki-67 labelling for the invasive carcinomas. There was a significant relationship between prominent reactivity and node status, all carcinomas with this degree of staining having metastasised. This, along with the differences between in situ and invasive carcinomas, suggests that TGF-β₁ may be a determining factor for invasion and metastasis.