The biological effects of cyclosporin, namely immunosuppression and absence of cytotoxicity, were discovered-in the course of a general screening program in which many scientists at Sandoz were involved. Contrary to some statements in the literature both Dr J. Borel and Dr H. Stähelin markedly contributed to the discovery and characterisation of the biological profile of the drug. In its subsequent exploitation Borel played the leading role. The outstanding clinical importance and the extraordinary commercial success of cyclosporin explain the widespread interest in the history of its discovery and development. It is also understandable that the recollection of the events by the individuals involved in the early phases of this history is influenced by subjective impressions and interpretations, which do not always reflect the historical facts. It is the purpose of this report to record and interpret the facts as accurately and as completely as possible on the basis of the available records.