Monocytes and granulocytes were incubated with suspensions of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, or Salmonella enteritidis and, after being washed free of bacteria, cultured for up to 48 h. Every few hours, samples of cultured cells were taken for DNA isolation. Monocytes which phagocytosed bacteria showed features of apoptotic cells, as determined by light microscopy and DNA fragmentation detected by gel electrophoresis. The phenomenon was observed 2 to 4 h after phagocytosis, in contrast, control monocytes did not show signs of apoptosis until 48 h of culture. Intact control granulocytes spontaneously became apoptotic after 12 h of culture. In contrast, degradation of DNA in cells exposed to bacteria was delayed by 12 to 24 h. In conclusion, our observation suggests that granulocytes and monocytes react differently to phagocytosis of bacteria.