Cordyceps sinensis (CS) is a parasitic fungus that has been used as a Chinese medicine for a long time in the treatment of nephritis. Today, the hypothesis about the pathogenesis of immunoglobulin A nephropathy (IgAN) is that nephritogenic IgA immune complexes (IgAIC) go to the kidney to stimulate resting mesangial cells to release cytokines and growth factors. These cytokines and growth factors cause mesangial cell proliferation and release matrix, chemical mediators that lead to the glomerular injury. However, nephritogenic IgAIC in humans is still unknown. To solve this problem previously, we established an in vitro model that showed that cultured human mesangial cells (HMC) stimulated with interleukin-1 (IL-1) plus IL-6 can cause mesangial cell proliferation, increasing production of chemical mediators and superoxide anion. An in vivo model also proved that this culture medium may lead to renal injury with hematuria and proteinuria. Therefore, to fractionate the crude components that can be used in the treatment of patients with IgAN, we cultured HMC, and then an HMC activating model with HMC incubated with IL-1 and IL-6 was established. We fractionated the crude methanolic extracts from fruiting bodies of CS with the use of this in vitro inhibition of HMC activation model as our assay method. In brief, the fruiting bodies were extracted by silica gel column chromatography. One out of 6 column fractions, F-2, significantly inhibited the HMC activation by IL-1 plus IL-6. The acute toxicity test with male Institute of Cancer Research mice showed no liver toxicity or mutagenicity. Then we established an IgAN animal model with R36A (Pneumococcal C-polysaccharide purified from Streptococcus pneumoniae) as antigen and anti-R36A IgA monoclonal antibody to form nephritogenic IgA-IC, which can induce hematuria and proteinuria in mice with IgA deposition in the mesangial area. The mice in the IgAN model fed with 1% F-2 in diet had significant reduction of hematuria and proteinuria together with histopathologic improvement. Therefore this fraction was then purified by silica gel column chromatography and high-performance liquid chromatography, which got a purified compound H1-A, which can suppress the activated HMC and alleviate IgAN (Berger's disease) with clinical and histologic improvement. These results give us a new regimen for the treatment of patients with IgAN in the future.