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A quantitative, repeatable, heterologous radioimmunoassay (RIA) for insulin-like growth factor-I (IGF-I) was developed for bovine serum. Untreated serum could not be assayed due to interference by IGF-I-binding protein. Serum acidified to pH 3·6 in glycine–HCl (0·1 mol/l) for 24 h at 37 °C and neutralized with NaOH produced inhibition curves non-parallel to the [Thr⁵⁹]-IGF-I standard. Neutralization by 40-fold dilution of acidified serum samples with assay buffer produced inhibition curves nearly parallel to the IGF-I standard. Complete parallelism was achieved by utilizing preprecipitated normal rabbit serum–sheep anti-rabbit -globulin to separate antibody-bound ¹²⁵I-labelled IGF-I from free ¹²⁵I-labelled IGF-I. Recovery of IGF-I (1·3–52·3 fmol) added to serum was quantitative. The sensitivity of the RIA (n = 6) was 8·25 ± 0·17 (S.E.M.) fmol. Intra- and interassay coefficients of variation were 3·03 and 4·95% respectively. Serum IGF-I levels measured in beef calves at weaning were positively correlated with weaning weight, total weight gain from birth to weaning and average daily weight gain. In conclusion, a heterologous RIA for IGF-I in beef serum which is sensitive, accurate, precise and repeatable has been developed.

J. Endocr. (1988) 119, 281–285

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