It is becoming increasingly clear that the inflammatory reaction can be ascribed to a complex array of mediators generated and released from activated phagocytes. In this study, the effect of PAF on interleukin-1(IL-1) activity by rat alveolar macrophages(AM) was examined using thymocyte proliferation assay in the supernate of sample obtained after 24 hr culture. When AM were cultured with PAF alone, no change in IL-1 activity was observed. However, the combined addition of PAF and muramyl dipeptide(MDP) or lipopolysaccharide(LPS) to AM cultures markedly enhanced IL-1 activity by 2-3 fold compared with AM cultures with the stimulant alone in a concentration dependent fashion. The peack effect was found at 10⁣8 M PAF with MDP and 10⁣14 M PAF with LPS. the effect of PAF was also tested in silica, toxic respirable dust, -added AM cultures as well as in the cultures containing bacterial compounds. Although silica did not stimulate the IL-1 activity, PAF could enhance IL-1 activity by 2 fold above the value of the silica-treated AM cultures with the peak response at 10⁣12 M PAF. Optimal enhancement of IL-1 activity occured when MDP and PAF were present together at the initiation of the 24 hr AM cultures. Additionaly, the biologically inactive precursor/metabolite of PAF, lyso-PAF failed to induce enhancement of IL-1 activity. When the specific, but structurally different PAF receptor antagonists, BN 52021(10⁣5 M) and CV 3988(10⁣5 M) was treated 15 min before addition of PAF(10⁣8 M) and MDP(10 ㄍg/ml) to the AM cultures, it markedly inhibited the enhancement of IL-1 activity induced by PAF. The effects of these PAF antagonists were also observed in LPS(1.0 ㄍg/ml)-stimulated cells. Collectively, these data suggest that PAF enhances IL-1 activity by interaction with a specific receptor.