A study has been made of the potential use of a terbium - transferrin complex as a non-isotopic label in the immunoassay determination of the antibiotic gentamicin. The fluorescence properties of the complex have been characterised. The labelled gentamicin was formed using a controlled carbodiimide reaction, conditions being chosen to produce a gentamicin-bound complex containing the correct amount of gentamicin for use in a competitive binding assay. Recognition of the gentamicin-bound complex by antisera to gentamicin was verified using a standard radioimmunoassay for gentamicin.