Details of the endocrine monitoring of patients during in-vitro fertilization are analysed. Hormones usually measured are 17 beta-oestradiol, FSH, LH, progesterone and HCG. The assays must be rapid, robust, and have satisfactory precision and reproducibility. Radioimmunoassays have become standardized but immunoradiometric assays are being introduced, where the antibodies can be more easily labelled than antigens, but need care with very high concentrations of antigen. Enzyme immunoassays also have advantages: they are less hazardous than radioimmunoassays and the labelled materials have longer shelf-lives. Chemiluminescence has also been introduced, together with time-resolved fluoroimmunoassays, representing simpler, perhaps cheaper and improved methods. Immunoassays by latex particle counting offer the advantage of being completely automated. Practical examples of these methods are given with details of the treatment of individual patients.