As well as its ability to induce cancer in various tissues, the polycyclic hydrocarbon 3-methylcholanthrene (MCA) is capable of affecting the activity of certain endocrine glands1–3 and altering the metabolism of several hormones4–7. Indirect evidence from several sources6,8–12 indicates that, in various experimental conditions, MCA may be capable of inhibiting, potentiating or mimicking the activity of certain steroid hormones. We have conducted preliminary experiments aimed at testing more rigorously the alleged steroidal activity of MCA. In the experiment reported here, MCA was tested for oestrogenic (uterotrophic) activity in a four point assay using oestrone as the reference standard.