An improved radioimmunoassay for serum gentamicin levels using 125I-labelled gentamicin.

1. A radioimmunoassay is described for the measurement of gentamicin in serum or plasma. 2. The assay technique uses a tracer labelled with iodine-125 rather than tritium, and has advantages over previously reported radioimmunoassay methods (Longmore et al., 1976; Lewis, Nelson & Elder, 1972) with regard to rapidity, precision and simplicity of preparation of labelled gentamicin. 3. The iodination technique is simple and gives tracer in high yield, at high specific activity, and with complete immunological identity to unlabelled gentamicin. 4. There is a significant correlation between results obtained by this technique and by a microbial assay method but the radioimmunoassay is more rapid, specific and accurate.