Platelet-activating factor(PAF) is a phospholipid mediator of pulmonary inflammation, and immunologic reaction. In this study, the role of PAF on tumor necrosis factor(TNF_-α) production by rat alveolar macrophages(AM) was examined. When PAF (10^-12 ~ 10^-16 M) alone was added to AM culture, (TNF_-α) production was not significantly increased above the resting level. In contrast, the combined addition of PAF (10^-6 M) and muramyl dipeptide(MDP) (1.0 μg\ml) to AM cultures markedly enhanced (TNF_-α) production with 8.2 fold increase compared with AM culture in resting state. This potentiative effect was 313% above the sum of the separate effects of PAF and MDP. To characterize MDP effects on (TNF_-α) production, the dose-response of AM cultured with various concentrations of MDP was tested. High level of MDP (10 μg\ml) could not significantly enhance the potentiation effect on (TNF_-α) production compared with AM cultures with low level of MDP (0.1 μg\ml), i.e. 112.5% vs 107.8%, respectively when 10^-10 M of PAF was simultaneously added to the cell culture. These data support that the potentiation of TNF. g production in AM culture is mediated by PAF rather than MDf It was also evaluated whether the similar result was obtained in silica, respirable toxic particle-treated AM culture. (TNF_-α) production was also significantly enhanced in the PAF (10^-6 M) and silica (50 μg\ml)-added cell cultures with 4.7 fold above the value of silica alone-stimulated cells. These results indicate that PAF can potentiate (TNF_-α) production by MDP-or silica- stimulated AM and suggest that PAF may play a potent role in lung inflammation and disease associated with microbe and occupational dust exposures.