In this study we examined the influence of various crude and recombinant cytokines on the ingestion and intracellular survival of Mycobacterium paratuberculosis within bovine monocytes and monocyte-derived macrophages. Cytokine pretreatment had little effect on the ingestion of M. paratuberculosis by bovine monocytes and macrophages. Monocytes that were continuously incubated with virus-induced crude bovine interferon (100 U) or recombinant bovine alpha interferon (100 U) significantly restricted the subsequent intracellular growth of M. paratuberculosis, as determined by microscopic counts of acid-fast bacilli and by recovery of CFU from lysed monocyte monolayers. In contrast to their effects on freshly adherent monocytes, these cytokines had little effect on the growth of M. paratuberculosis within monocyte-derived macrophages. In two separate experiments, we also observed inhibition of bacillary growth in monocytes treated with unpurified recombinant human granulocyte-macrophage colony-stimulating factor. Conversely, intracellular growth of M. paratuberculosis was enhanced in monocytes that were pretreated with culture supernatants from M. paratuberculosis-stimulated peripheral blood mononuclear cells obtained from an immunized calf. The growth-enhancing activity of these supernatants was labile at pH 2.0, suggesting a role for gamma interferon; however, subsequent experiments indicated that recombinant gamma interferon alone neither enhanced nor restricted intracellular bacillary growth. To determine the possible contributions of monocyte oxidative activity to cytokine-induced bacteriostasis, we compared the release of superoxide anion from cytokine-treated and control monocytes. No obvious relationship was observed between the release of superoxide anion and the subsequent intracellular fate of the bacilli.