Three new immunoassays for determination of serum gentamicin concentration were assessed: a latex agglutination assay (Technicon), an enzyme immunoassay (EMIT 2000) and a multi-layer thin-film immunoassay (Opus). All three were easy to use, had good within- and between-assay reproducibility, good reliability and acceptable linearity. The standard curve stability was shortest with the latex agglutination assay and longest with the thin-film. The thin-film assay calibrators gave approximately double the expected concentration when assayed by polarisation fluoroimmunoassay but this did not effect the assay's performance. When compared with the polarisation fluoroimmunoassay results of patient samples, all three methods gave an excellent correlation. The technical aspects of these three methods make them acceptable for clinical use.