Of the two common morphotypes of Mycobacterium avium, designated smooth transparent (SmT) or smooth opaque (SmO), the SmO morphotype is avirulent, whereas the SmT morphotype is virulent. The role of the host macrophage in determining these different virulence phenotypes was analyzed using an in vitro model of macrophage infection. Initial studies confirmed previous reports of the increased ability of the SmT bacteria to grow in macrophages; this increased virulence correlated with reduced induction of inflammatory cytokines. Examination of the response of the mitogen-activated protein kinase (MAPK) pathway following infection with either morphotype revealed that all three members of the MAPK pathway were activated. Pharmacologic inhibition of either the extracellular signal-regulated kinase (ERK) or p38MAPK pathways resulted in distinct consequences for the growth of the two morphotypes. In particular, inhibition of the p38MAPK resulted in attenuated growth of the SmT morphotype, which correlated with reduced PGE2 production. Inhibition of cyclooxygenase 2 by indomethacin also inhibited growth of SmT, substantiating the role for PGE2 in promoting the growth of SmT. In contrast, SmO induction of the ERK pathway was increased compared with the SmT morphotype, and inhibition of ERK resulted in decreased TNF-α synthesis and enhanced SmO growth. Pharmacologic inhibitors of the MAPK pathway were present for only the first 4 h of infection and yet had consequences for bacterial growth at 7 days. Therefore, the data suggest that induction of the MAPK pathway during uptake of bacteria is instrumental in determining the eventual fate of the bacteria.