We previously reported that IFN-gamma and LPS can stimulate bovine monocytes to produce nitric oxide, which is blocked by addition of NMMA. The purposes of the present study were to determine the effect of IFN-gamma on the interaction of M. avium ss paratuberculosis with bovine monocytes, and to find out whether this effect was related to production of nitric oxide by activated bovine monocytes. Freshly isolated peripheral blood monocytes were cultured with different concentrations of IFN-gamma, or NMMA, for 16 to 18 hours, and infected with M. avium ss paratuberculosis. At the time of infection, LPS was added to some IFN-gamma pretreated monocytes and monocytes cultured with NMMA. Monocytes were then cultured up to 12 days after infection and then lysed with 0.25% SDS at 0, 3, 6, 9 and 12 days after infection. Radiometric counting was used to determine the numbers of viable intracellular M. avium ss paratuberculosis. Monocytes incubated with IFN-gamma exhibit a trend towards slightly increased phagocytosis of and inhibition of intracellular M. avium ss paratuberculosis by bovine monocytes. This effect was not altered by addition of LPS and NMMA to IFN-gamma pretreated monocytes. These data suggest that the amount of nitric oxide produced by bovine monocytes may be insufficient to inhibit intracellular growth of M. avium ss paratuberculosis.