Lysis and precipitation of polyoxyethylene sorbitan monoleate (Tween 80) was used as an indicator of growth in quantitative antibiotic dilution susceptibility tests in microtiter plates. This allowed test results to be read after 2 or 3 days. Antibiotic sensitivity levels obtained with this method were homogenous and tightly clustered. No acquired resistance was found among 16 field strains isolated in 1990.