Bovine monocytes exhibited a greater ability to phagocytose Mycobacterium avium subsp. paratuberculosis (i.e. greater percentage of infected cells, and more bacilli per infected cell), than did a bovine macrophage cell line (BoMac). Phagocytosis of M. paratuberculosis by monocytes, but not the cell line, was significantly enhanced by the addition of autologous serum. Following ingestion, the numbers of viable M. paratuberculosis cells in monocytes increased during the first 4 days and then declined between day 4 and day 8 after infection, as determined by a radiometric method. In contrast, BoMac cells were not permissive for bacillary multiplication; the numbers of M. paratuberculosis remained largely unchanged in the cell line during the 8 day incubation period. The numbers of microscopically visible acid-fast bacilli increased with time in monocytes but not in the macrophage cell line. These observations suggest that replication and inactivation of bacilli may both occur in monocytes. The differing abilities of bovine monocytes and the macrophage cell line to ingest and restrain the intracellular growth of M. paratuberculosis provide contrasting model systems for investigating how M. paratuberculosis enters and persists within its preferred niche, the mononuclear phagocyte.