As part of studies undertaken to investigate the pathogen reducing capabilities of conventional sludge stabilization procedures, microbial reductions produced by mesophilic and thermophilic digestion at the Los Angeles Hyperion Plant were examined. Samples from the digester feed and product streams were collected over a 2-year period and sent to EPA's Cincinnati Laboratory for bacterial, viral, and parasite determination. These results were compared with results from two Ohio treatment plants. From the Los Angeles sludge data, reductions in certain bacteria and viruses by mesophilic anaerobic digestion can be defined--as can defensible confidence limits. These data could not be supplemented with information on bacterial reductions produced by other digesters because the densities of Salmonella sp. in the influent of these other digesters were not sufficiently high.