Adsorption coupled with electrostatic interaction as an immobilization technique is an important microbial cell immobilization technique. Treatment of the polymer matrix with the cationic surface treating agent chitosan for lactic acid production has been studied. Cells of Lactobacillus plantarum NCIM 2084 were immobilized on a polypropylene (PP) matrix treated with different concentrations of chitosan. The biocatalyst adsorbed on the 1.0 g dm(-3) chitosan-treated PP matrix proved to be most effective. Repeated batch fermentation experiments showed that the immobilized biocatalyst could be recycled effectively ii times. Studies were also carried out in a packed bed reactor with media recirculation. A high productivity of 7.66 g dm(-3) h(-1) could be obtained with a conversion of 94% and a yield of 97% at an average residence time of 30 h.