A radioimmunoassay method for the assay of luteinising hormone (LH) and follicle-stimulating hormone (FSH) in serum/plasma has been designed for use in laboratories of varying expertise in the United Kingdom. The major sources of experimental error leading to poor within-laboratory performance and between-laboratory comparability were identified: quality of tracer, use of calibration standards, and separation procedure. A simple rugged kit was designed which was extensively tested first in our laboratory and then in a small multi-centre field trial before being made available. It is now used routinely by 26 health service and research laboratories. The working range of the assays is 1-50 IU/l (LH) and 0.3-16 IU/l (FSH). The between-batch reproducibility was 5-11% (CV) over the dose range 4.8-18 IU/l (LH) and 1.6-15 IU/l (FSH).