Voices of the moon.

Professor Geoffrey Chamberlain, FRCOG, is chairman of the department of obstetrics and gynaecology at St George's Hospital Medical School, London. Enzyme linked immunosorbant assays (ELISAs) are the bases of many of the commercial kits currently available in chemist shops. The assay depends on the double reaction of standard phase antibody with enzyme labelled antibody, which is sensitive and so able to detect very low concentrations of human chorionic gonadotrophin. Positive results are therefore detectable as early as 10 days after fertilisation-that is, four days before the first missed period.