A competitive enzyme immunoassay for rat growth hormone (rGH) has been developed using polyclonal anti-rGH antibodies and an acetylcholinesterase (EC 3.1.1.7.) enzymatic tracer coupled covalently with rGH. The assay was performed in 96-well microtiter plates coated with rabbit polyclonal anti-goat immunoglobulin antibodies. Molecular sieve filtration and Western blot analysis revealed a single immunoreactive peak for rat plasma or pituitary extracts. Cross-reactivity with other rat pituitary hormones or human GH was less than 1%. Assay of samples in a concentration range of 0.7 to 69 ng/ml by enzyme immunoassay and radioimmunoassay were well correlated (r = 0.87 and 0.85 respectively for plasma and culture medium samples). Intra- and inter-assay variations in plasma were 4 (n = 24) and 14% (n = 9) respectively. Minimal detectable amounts of rGH were 0.6 ng/ml. A two-site immunometric assay also developed with the same antibodies allowed a detection threshold of 0.25 ng/ml.