In vitro phagocytosis and intracellular survival of Campylobacter jejuni strain 2964 in mononuclear phagocytes were studied. The following three types of mononuclear phagocytes were used: a J774G8 peritoneal macrophage line derived from BALB/c mice, resident BALB/c peritoneal macrophages, and human peripheral blood monocytes. When C. jejuni and mononuclear phagocytes were combined at a ratio of 75:1, light microscopy, fluorescent microscopy, and electron microscopy all indicated that C. jejuni cells were readily phagocytized. The majority of C. jejuni cells were spirals immediately following ingestion and were rapidly converted to the coccal form within 4 to 8 h. Conversion from the spiral form to the coccal form was complete in the presence of phagocytes within 96 h. In control preparations without phagocytes, conversion began after 24 h and was complete after 48 h. The extent of phagocytosis over time was determined by observing Giemsa-stained preparations and counting the number of intracellular bacterial colony-forming units after removal of extracellular C. jejuni. Human monocytes ingested C. jejuni more rapidly and vigorously than murine macrophages. Intracellular survival of C. jejuni was examined by measuring the number of C. jejuni colony-forming units associated with phagocytes after phagocytosis for 2 h and removal of extracellular bacteria. C. jejuni survived intracellularly for up to 6 to 7 days.