An array of endophytic fungi which habitat in a Chinese medicinal plant, Tripterygium wilfordii Hook. f., (TWHf) were isolated and tested for their suppressive activity on phytohemaglutinin (PHA) stimulated proliferation of human peripheral blood mononuclear cells (PBMC). Out of the 343 isolates, representing 60 different morphotypes, were screened. Amongst the screened fungi, only 15 isolates showed anti-proliferative activity. Ethyl acetate extracts of Pestalotiopsis leucothës, Mucor sp. Verticillium sp. or Pestalotiopsis disseminata, in particular, significantly inhibited the proliferation at doses between 0.12 and 500 microg/ml (stimulation index (S.I.) ranges 0.01-0.70) (P < 0.001-0.05). IC50 values of these four fungal extracts were between 0.75-0.80 +/- 0.12 microg/ml. Trypan blue exclusion test and visual examination of cell morphology demonstrate that the anti-proliferative effect of these extracts was not a sequential consequence of their cytotoxicity. Nevertheless, fungal extracts of Acremonium sp. A and C, Pestalotiopsis suffocata, Morphotype sp. 4 and 5 show a much higher cytotoxic effects on PBMC. Our results indicate that some fungal extracts contain significant amount of immunomodulatory principles.