Phagocytosis by human leukocytes, phagosomal pH and degradation of seven species of bacteria were studied by a flow cytometric method. The percentage of phagocytosing leukocytes was similar for all bacterial strains examined, but Salmonella typhi and Neisseria meningitidis were more slowly phagocytosed than other bacteria. The phagosomal pH surrounding the different bacterial species 15 min after the start of phagocytosis were: Streptococcus pneumoniae 4.4; N. meningitidis 4.9; Str. pyogenes 5.1; Staphylococcus aureus 5.2; Escherichia coli 5.3; S. typhi 5.4; and Klebsiella pneumoniae 5.7. For longer incubation periods, the phagosomal pH remained nearly constant. Staph. aureus, E. coli and S. typhi were the most readily degraded of the species tested. The proteins of all bacteria were degraded more rapidly than their DNA as determined by measurements of the loss of fluorescein-isothiocyanate-fluorescence and ethidium bromide-fluorescence, respectively. The rate of degradation varied from one bacterial species to another. The degradation of proteins and DNA was maximal for bacteria residing in a phagosomal environment estimated to be between pH 5.2 and 5.4.