We studied the expression of interferon alpha (IFN alpha)-mRNA in porcine non-adherent peripheral blood mononuclear cells (PBMC) after induction by the coronavirus transmissible gastroenteritis virus (TGEV). We found that protein synthesis inhibition by cycloheximide (CHX) blocked IFN alpha-mRNA expression, except when PBMC were preincubated with a conditioned medium as a potential source of cytokines. These data indicate that IFN alpha-mRNA induction by TGEV requires de novo synthesis of proteins. Moreover, they suggest that IFN alpha-mRNA induction in porcine leukocytes by TGEV involves mechanisms identical to those described for the herpes simplex virus in humans. In addition, experiments performed with a TGEV mutant, dm 49-4, previously characterized for its low ability to induce IFN alpha, showed that addition of a conditioned medium could not normalize its IFN alpha-inducing ability. Therefore, the defect of the dm49-4 mutant may be at the level of the final triggering signal to PBMC.