Phosphodiesterase 4 (PDE4) inhibitors elevate cyclic adenosine 5′-monophosphate (cAMP), and this elevation has been shown to inhibit inflammatory cytokines such as tumor necrosis factor-α (TNF-α). Using TNF-α as a biomarker, we have developed transcription-based assays to examine inhibition of PDE4 activity in human and guinea pig whole blood. In vitro inhibition by PDE4 inhibitors was measured using quantitative PCR (qPCR) analysis of TNF-α mRNA levels in whole blood stimulated with lipopolysaccharide (LPS). The kinetics of human TNF-α mRNA production were analyzed and shown to be highest 4hr following LPS stimulation. The guinea pig displayed kinetics of TNF-α transcription similar to those of the human. Analysis of inhibition of human TNF-α protein production was performed by immunoassay and shown to correlate with inhibition of transcription for three of the four compounds tested. Roflumilast was found to be 9-fold more potent for TNF-α inhibition in the qPCR assay than in the protein assay. The potencies of L-826,141 and roflumilast were determined in human and guinea pig whole blood by qPCR, with ic₅₀ values of 270 and 20nM, respectively, in humans and 100 and 10nM, respectively, in guinea pigs. These results show that the potency of PDE4 inhibitors can be monitored in whole blood using a transcription-based assay, and that this type of assay can be adapted to various species provided the TNF-α nucleotide sequence is known. The in vitro whole blood ic₅₀ for TNF-α inhibition was compared to inhibition in the ovalbumin-challenged guinea pig model of bronchoconstriction. Obtaining plasma levels at the ic₅₀ determined in vitro for L-826,141 and roflumilast provides significant inhibition of bronchoconstriction. This suggests that TNF-α can be used as a whole blood biomarker in the guinea pig for PDE4 inhibition in this inflammatory model.