The fluorescence polarization immunoassay technique has been used previously for the assay of vancomycin and the aminoglycoside antibiotics (gentamicin, tobramycin, and amikacin). We extended this technique to assay streptomycin. Fluorescein-labeled streptomycin was used as the tracer, and antiserum specific for streptomycin was raised in rabbits by conventional procedures. Tracer, sample, and diluted antiserum were combined, and the polarization of tracer fluorescence was determined in a specially designed fluorometer (Abbott TDx). Because of the instrument design, the possibility of fluorescent interferences was eliminated. The coefficient of variation within run was 4% (n = 5), and between run it was 5% (n = 5). We compared the fluorescence polarization immunoassay technique (TDx streptomycin) with a conventional bioassay, using Bacillus subtilis for clinical specimens (n = 39). A least-squares linear regression analysis gave a slope of 1.16, an intercept of 2.41 mg/liter, and a correlation coefficient of 0.885. Repeat analyses by both techniques showed that the largest discrepancies could be explained by the imprecision of the bioassay.