The development of a competitive enzyme immunoassay for bovine growth hormone (bGH) is described. Antiserum, raised in rabbits and diluted 1/250,000, was preincubated with samples and free antibodies from the reaction mixture were immobilized using a microtiter plates coated with bGH. Bound antibodies remaining from the preincubation were visualized using a biotinylated second antibody as a bridge for subsequent amplification by an avidin-biotin-peroxidase complex. The measuring range was between 0.5 and 100 ng/ml. Cross-reactivity with other pituitary hormones was < 0.1%. The intra- and interassay coefficients of variations were 8.1 and 12.7%, respectively, and the recovery of added bGH was 110%. To validate the assay, two bull calves were treated with bGH-releasing factor. The response showed that there was an immediate rise in immunoreactive bGH which peaked after 5 and 15 min at 200 ng/ml and 60 ng/ml, respectively. This enzyme immunoassay is an economic and sensitive alternative to the established radioimmunoassay, and the first competitive enzyme immunoassay described for bGH.