Aerobically activated sludge from a pig slaughterhouse wastewater-treatment plant was mixed with 0.35 or 0.5% (wt/wt) dextrose, with 0.5% (wt/wt) dextrose and 0.05 or 0.2% (v/v) formic acid, or with 2.5% (wt/wt) molasses and 0.1% (v/v) formic acid, and was pasteurized at 95 degrees C for 5 min. After cooling to approximately 20 degrees C the sludge was inoculated with 10(6)-10(7) cfu Lactobacillus plantarum/g sludge. Fermentation was performed at 20 degrees C for 21 days. When 0.05% (v/v) formic acid or less was added the initial pH of the sludge was favourable for the germination and outgrowth of clostridia spores and resulted in amino acid breakdown. In the case of the addition of a 2% (v/v) formic acid and 0.5% (wt/wt) dextrose the initial pH was below 4 and the number of lactobacilli did not substantially increase. No amino acid breakdown was observed. Concerning the safety of the fermented product, it was concluded that pasteurized activated sludge from a pig slaughterhouse can be effectively fermented into a stable product, suitable for animal feed purposes, with 2.5% (wt/wt) molasses, 0.1% (v/v) formic acid and L. plantarum as the inoculum.