Müllerian inhibiting substance (MIS), or antimüllerian hormone, is produced by testicular Sertoli cells in fetal (and later) life causing regression of the müllerian duct system. In males with normal testes its serum concentration is high at birth and decreases thereafter. In females it is produced by ovarian granulosa cells at and after puberty. Concentrations in adult men and women are similar.  In children an appreciable serum MIS concentration indicates the presence of testicular tissue. Now researchers in Boston, Chicago, and Tokyo (Mary M Lee and colleagues, New England Journal of Medicine 1997;336:1480-6; see also editorial, Ibid: 1519-21) have shown serum MIS measurement to be useful in the assessment of virilised children with no palpable gonads. They tested 65 children with a variety of diagnoses including cryptorchidism, anorchia, gonadal dysgenesis, and adrenogenital syndrome. Mean (SD) concentrations were 48.2 (42.1) ng/ml in 34 children with normal testes, 11.5 (11.8) in 14 with abnormal testes, and 0.7 (0.5) in 17 with no testes. The test was 92% sensitive and 98% specific for predicting the absence of testicular tissue, an improvement on tests using testosterone measurements.  The test may prove useful in assessing boys with undescended testes and children with ambiguous genitalia but it is not yet widely available.