Endotoxin (LPS), one of the major proinflammatory constituents of the cell walls of gram-negative bacteria, induces alveolar macrophages to express interleukin-1 (IL-1) and tumor necrosis factor (TNF) messenger RNA (mRNA), peaking at 1 hour in vitro. Intratracheal injection of LPS induces IL-1 and TNF mRNA expression in vivo in whole-lung RNA preparations. Interleukin-1 mRNA is not constitutively detected. In the case of TNF, however, a constitutively-expressed hybridization band is noted at 1.6 kb, whereas the LPS-induced hybridization band is noted at approximately 1.95 kb. Intratracheal injection of LPS induces an intra-alveolar inflammatory reaction composed of a neutrophilic exudate, peaking at 6 to 12 hours, a monocytic exudate peaking at 24 hours, and a lymphocytic exudate peaking at 48 hours, as quantitated by bronchoalveolar lavage. Intratracheal injection of IL-1 recapitulates the kinetics and relative magnitudes of the acute neutrophilic and chronic monocytic and lymphocytic inflammatory sequence. Intratracheal injection of TNF also induces an acute intraalveolar neutrophilic exudate, but TNF is much less potent of an inflammatory stimulus than IL-1. The effects of recombinant IL-1 and TNF are not due to LPS contamination, as shown by abrogation of the cytokines' inflammatory activity by boiling. In conclusion, LPS induces IL-1 and TNF mRNA expression in vitro in alveolar macrophages and in vivo in pulmonary tissue, and intratracheal injection of IL-1 and TNF recapitulates the LPS-induced pulmonary inflammatory sequence, strongly supporting the hypothesis that these cytokines play an important in vivo role in the pathogenesis of gram-negative bacterial pneumonia.