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The intracellular activity of cefamandole and aztreonam against phagocytosed Escherichia coli and cefamandole against phagocytosed Staphylococcus aureus was studied using a sensitive and standardized method of murine peritoneal macrophages. Cefamandole and aztreonam exerted an intracellular antibacterial activity against E. coli which was greater than their extracellular one. With concentrations of both antibiotics up to 16 x MBC a dose-dependent decrease of the initial number of intracellular E. coli which ranged from 32% to 90% was observed. However, similar antibiotic concentrations above the MBC affected the viability of extracellular E. coli by only 20% to 30%. The intracellular antibacterial activity of both antibiotics against E. coli was further enhanced by immune serum. Cefamandole at 4 x the MBC did not affect the survival of intracellular S. aureus, but killed 41% of extracellular bacteria by 1 h and 99% after 3 h. The intracellular activity of both antibiotics against E. coli was also maintained in NaF-pulsed macrophages which have an impaired oxidative metabolism. The data suggest that both cefamandole and aztreonam possess an intracellular antibacterial activity against E. coli that seems at least in part due to a positive cooperation of antibiotics with the O2-independent microbicidal system of macrophages. [ABSTRACT FROM AUTHOR]