Bacterial antibiotic susceptibility was rapidly estimated for Escherichia coli and Staphylococcus spp. by flow cytometry. This was achieved by measuring the uptake of a negatively charged membrane potential sensitive dye bis-(1,3-dibutyl-barbituric acid) trimethine oxonol and observing changes in low-angle light scatter (excitation light scattered by up to 15 degrees). Estimations of ampicillin, gentamicin and ciprofloxacin susceptibilities were possible within 2-5 h from a plate culture, depending on the species and antibiotic used. This includes the time necessary to establish steady-state growth in liquid culture.