Many patients with advanced non-thyroid malignancies have elevated plasma immunoreactive calcitonin concentrations. Breast and bronchial carcinomas contain immunoreactive calcitonin and an epidermoid bronchial carcinoma has been shown to produce immunoreactive calcitonin in vitro. We have established monolayer cultures of breast carcinomas and eight out of fifteen consecutive carcinomas released immunoreactive calcitonin; some released HCG (human chorionic gonadotrophin) or CEA (carcinoembryonic antigen). In addition, a primary human breast carcinoma has been shown to release and contain calcitonin after being passaged in 'nude' mice over 1 year. Chromatography of extracts and culture media of a bronchical carcinoma demonstrated that, in contrast with the other tumours, it secreted a form or forms of calcitonin having size, charge and immunological differences when compared to calcitonin M. Preliminary evaluation of plasma immunoreactive calcitonin estimations in patients with breast carcinoma showed that twenty-three out of twenty-eight patients with metastatic disease had elevated plasma calcitonin concentrations, whereas only one out of thirteen with localized disease had high levels.