Pseudolaric acid B (PAB) is a major bioactive component of the medicinal plant Pseudolarix kaempferi. Traditional medicine practitioners in Asia have been using the roots of this plant to treat inflammatory and microbial skin diseases for centuries. In the current study, in vitro immunosuppressive effect of PAB and the underlying mechanisms have been investigated. The results showed that PAB dose-dependently suppressed human T lymphocyte proliferation, IL-2 production and CD25 expression induced by co-stimulation of PMA plus ionomycin or of anti-OKT-3 plus anti-CD28. Mechanistic studies showed that PAB significantly inhibited nuclear translocation of NF-kappaB p65 and phosphorylation and degradation of IkappaB-alpha evoked by co-stimulation of PMA plus ionomycin. PAB could also suppress the phosphorylation of p38 in the MAPKs pathway. Based on these evidences, we conclude that PAB suppressed T lymphocyte activation through inhibition of NF-kappaB and p38 signaling pathways; this would make PAB a strong candidate for further study as an anti-inflammatory agent.