The effect of the addition of amino acids on the microfiltration behavior of Escherichia coli (E coli) and Corynebacterium glutamicum (C glutamicum) slurry was examined using a dead-end microfilter. It was found that the average specific filtration resistance, alpha(av), of a slurry of microorganisms increased markedly by adding amino acids. Amino acid concentration ranged from 110 to 876 mol m(-3). However the same concentration of ammonium chloride did not increase alpha(av). The cells were found to disperse in the presence of lysine, and this caused an increase in alpha(av). In the case of kaolin slurry, alpha(av) was not affected by adding amino acids. (C) 2004 Society of Chemical Industry.