Insulin exists in dilute aqueous solutions as, a relatively stable globular structure, the maintenance of which appears to be important in receptor recognition. A similar conformation is probably adopted by the homologous insulin-like growth factor, which can exhibit insulin-like activity but which does not bind to anti-insulin antibodies. Relaxin, a hormone from the corpus luteum involved in relaxation of the pubis symphysis prior to parturition, may also have an insulin-like tertiary structure but has no insulin-like activity. © 1979.