The hepatic synthesis of acute phase proteins in ruminants has been suggested to be regulated by some mechanisms different from those in other species such as rodents and human. To explore possible fegulatory factors unique to ruminants, we examined effects of interleukin (IL)-6, IL-1 and tumor necrosis factor (TNF), on haptoglobin (Hp) synthesis using a primary culture system of bovine hepatocytes. After bovine primary cultured hepatocytes were incubated in the presence of various concentrations of the cytokines, the synthesis and mRNA level of haptoglobin and albumin were measured by labeling with [<35>S]-methionine and immunoprecipitation, and by Northern blot analysis, respectively. Hp synthesis was dose-dependently increased by recombinant human (rh) IL-6, and also by rhTNF-α, but to a less extent, while it was not affected by rhIL-1β. The stimulatory effect is mainly pretranslational, because mRNA level of Hp changed in parallel with protein synthesis. In contrast, albumin synthesis was suppressed by these three cytokines similarly. These results are inconsistent with the previously proposed view that TNF and IL-1 overlap in their pathways leading to the transcriptional activation of many acute phase protein genes. In conclusion, there is a species-specific unique signaling system, especially for TNF, in transcriptional activation of bovine Hp gene.