Measurements of transforming growth factor alpha (TGF-alpha) in cancer patients have produced variable results. We have now used a specific radioimmunoassay (RIA) and a mitogenic assay to evaluate TGF-alpha content of tumour and urine samples separated by an analytical HPLC system. Urine samples from patients with breast tumours and from age matched controls gave TGF-alpha amounts ranging from 0 to 61.5 ng 24 h-1 compared to urogastrone epidermal growth factor figures of 3.0-26.2 micrograms 24 h-1. The quantities of TGF-alpha in patient and control groups were not significantly different. The majority of breast tumour extracts contained mitogenic material eluting from the HPLC system at the TGF-alpha calibration point. Measurement by RIA of combined samples from each group showed that steroid receptor positive tumours had a mean figure of 14.8 ng g-1 tissue and steroid receptor negative 7.4 ng g-1. Receptor positive tumours from patients treated with an antioestrogen, tamoxifen citrate (Nolvadex), had 0.16 ng g-1. Thus TGF-alpha is found in tumours as a biologically active entity and in quantities sufficient to promote cell division. In addition the observation that tamoxifen causes a significant reduction in the content of TGF-alpha may be an additional beneficial action.