Impedimetric measurements were used to assay the antibacterial effect of protamine. A good linear correlation between the impedance detection time and the initial cell counts was obtained (r = 0.99, n = 2). As basic peptides may cause clumping of cells, this correlation curve was used when estimating the cell number after protamine treatment, rather than colony counts. Protamine from salmon killed growing Gram-positive bacteria and significantly inhibited growth of Gram-negative bacteria in Tryptone Soy Broth (TSB) at 25 degrees C. In general Gram-positive bacteria were more sensitive to protamine than Gram-negative bacteria; the minimum inhibitory concentrations (MIC) determined for Gram-positive strains varied from 20 to 1000 micrograms ml-1 and for Gram-negative strains from 500 micrograms ml-1 to more than 4000 micrograms ml-1. The effect of protamine on non-growing Listeria monocytogenes Scott A suspended in buffer was not lethal as was the effect on growing cells; however, protamine (50-500 micrograms ml-1) killed the Gram-negative fish spoilage bacteria Shewanella putrefaciens when the live cells were suspended in buffer.