A double-antibody-sandwich, enzyme-linked immunosorbent assay was developed to detect an extracellular proteinase produced by Pseudomonas fragi. The method was capable of detecting 4 mu g/ml of the proteinase in spiked samples of buffer and broth and 4.2 mu g/ml in a broth culture of the organism. The assay detected the presence of proteinase at bacterial densities of approximately 10(4) cfu/ml, which develop after incubation for 15 h at 25 degrees C in a broth medium. All assays could be completed within 7 h. This assay is of value in plotting proteolytic expression in relation to the growth cycle of Ps. fragi in broth culture and may be of value, with development, in other more complex milieux.